Parallel execution ran into "patch does not apply"
I modified a bunch of repos using `pre-commit` in parallel and ran `git commit` at the same time, with unstaged changes. The pre-commit processes did `[WARNING] Unstaged files detected.`, stashed the changes in a pach, ran, and then tried to reapply the patches.

Some repos failed with:


```
[WARNING] Stashed changes conflicted with hook auto-fixes... Rolling back fixes...
An unexpected error has occurred: CalledProcessError: command: ('/usr/local/Cellar/git/2.31.1/libexec/git-core/git', '-c', 'core.autocrlf=false', 'apply', '--whitespace=nowarn', '/Users/chainz/.cache/pre-commit/patch1618586253')
return code: 1
expected return code: 0
stdout: (none)
stderr:
    error: patch failed: .github/workflows/main.yml:21
    error: .github/workflows/main.yml: patch does not apply

Check the log at /Users/chainz/.cache/pre-commit/pre-commit.log
```

It looks like this is due to use of the unix timestamp as the only differentiator in patch file paths, causing the parallely-created patches to clobber each other.

`pre-commit.log` says:

### version information

```
pre-commit version: 2.12.0
sys.version:
    3.9.4 (default, Apr  5 2021, 01:49:30) 
    [Clang 12.0.0 (clang-1200.0.32.29)]
sys.executable: /usr/local/Cellar/pre-commit/2.12.0/libexec/bin/python3
os.name: posix
sys.platform: darwin
```

### error information

```
An unexpected error has occurred: CalledProcessError: command: ('/usr/local/Cellar/git/2.31.1/libexec/git-core/git', '-c', 'core.autocrlf=false', 'apply', '--whitespace=nowarn', '/Users/chainz/.cache/pre-commit/patch1618586253')
return code: 1
expected return code: 0
stdout: (none)
stderr:
    error: patch failed: .github/workflows/main.yml:21
    error: .github/workflows/main.yml: patch does not apply
    
```

```
Traceback (most recent call last):
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/staged_files_only.py", line 20, in _git_apply
    cmd_output_b('git', *args)
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/util.py", line 154, in cmd_output_b
    raise CalledProcessError(returncode, cmd, retcode, stdout_b, stderr_b)
pre_commit.util.CalledProcessError: command: ('/usr/local/Cellar/git/2.31.1/libexec/git-core/git', 'apply', '--whitespace=nowarn', '/Users/chainz/.cache/pre-commit/patch1618586253')
return code: 1
expected return code: 0
stdout: (none)
stderr:
    error: patch failed: .github/workflows/main.yml:21
    error: .github/workflows/main.yml: patch does not apply
    

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/staged_files_only.py", line 68, in _unstaged_changes_cleared
    _git_apply(patch_filename)
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/staged_files_only.py", line 23, in _git_apply
    cmd_output_b('git', '-c', 'core.autocrlf=false', *args)
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/util.py", line 154, in cmd_output_b
    raise CalledProcessError(returncode, cmd, retcode, stdout_b, stderr_b)
pre_commit.util.CalledProcessError: command: ('/usr/local/Cellar/git/2.31.1/libexec/git-core/git', '-c', 'core.autocrlf=false', 'apply', '--whitespace=nowarn', '/Users/chainz/.cache/pre-commit/patch1618586253')
return code: 1
expected return code: 0
stdout: (none)
stderr:
    error: patch failed: .github/workflows/main.yml:21
    error: .github/workflows/main.yml: patch does not apply
    

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/staged_files_only.py", line 20, in _git_apply
    cmd_output_b('git', *args)
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/util.py", line 154, in cmd_output_b
    raise CalledProcessError(returncode, cmd, retcode, stdout_b, stderr_b)
pre_commit.util.CalledProcessError: command: ('/usr/local/Cellar/git/2.31.1/libexec/git-core/git', 'apply', '--whitespace=nowarn', '/Users/chainz/.cache/pre-commit/patch1618586253')
return code: 1
expected return code: 0
stdout: (none)
stderr:
    error: patch failed: .github/workflows/main.yml:21
    error: .github/workflows/main.yml: patch does not apply
    

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/error_handler.py", line 65, in error_handler
    yield
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/main.py", line 357, in main
    return hook_impl(
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/commands/hook_impl.py", line 227, in hook_impl
    return retv | run(config, store, ns)
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/commands/run.py", line 408, in run
    return _run_hooks(config, hooks, args, environ)
  File "/usr/local/Cellar/python@3.9/3.9.4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/contextlib.py", line 513, in __exit__
    raise exc_details[1]
  File "/usr/local/Cellar/python@3.9/3.9.4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/contextlib.py", line 498, in __exit__
    if cb(*exc_details):
  File "/usr/local/Cellar/python@3.9/3.9.4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/contextlib.py", line 124, in __exit__
    next(self.gen)
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/staged_files_only.py", line 93, in staged_files_only
    yield
  File "/usr/local/Cellar/python@3.9/3.9.4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/contextlib.py", line 124, in __exit__
    next(self.gen)
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/staged_files_only.py", line 78, in _unstaged_changes_cleared
    _git_apply(patch_filename)
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/staged_files_only.py", line 23, in _git_apply
    cmd_output_b('git', '-c', 'core.autocrlf=false', *args)
  File "/usr/local/Cellar/pre-commit/2.12.0/libexec/lib/python3.9/site-packages/pre_commit/util.py", line 154, in cmd_output_b
    raise CalledProcessError(returncode, cmd, retcode, stdout_b, stderr_b)
pre_commit.util.CalledProcessError: command: ('/usr/local/Cellar/git/2.31.1/libexec/git-core/git', '-c', 'core.autocrlf=false', 'apply', '--whitespace=nowarn', '/Users/chainz/.cache/pre-commit/patch1618586253')
return code: 1
expected return code: 0
stdout: (none)
stderr:
    error: patch failed: .github/workflows/main.yml:21
    error: .github/workflows/main.yml: patch does not apply
```
diff --git a/pre_commit/staged_files_only.py b/pre_commit/staged_files_only.py
index 6179301..48cc102 100644
--- a/pre_commit/staged_files_only.py
+++ b/pre_commit/staged_files_only.py
@@ -47,7 +47,7 @@ def _unstaged_changes_cleared(patch_dir: str) -> Generator[None, None, None]:
         retcode=None,
     )
     if retcode and diff_stdout_binary.strip():
-        patch_filename = f'patch{int(time.time())}'
+        patch_filename = f'patch{int(time.time())}-{os.getpid()}'
         patch_filename = os.path.join(patch_dir, patch_filename)
         logger.warning('Unstaged files detected.')
         logger.info(f'Stashing unstaged files to {patch_filename}.')
['pre_commit/staged_files_only.py']