ValueError: path is on mount 'C:', start on mount 'E:'
## Reproduce

Probably only happens on Windows due to having a problem when calculating relative path between `C:` and `E:`.

Steps:
- Enable SL's debug mode.
- Open a ST project which is on `E:`.
- Open a file on `C:`.
- The opened file from `C:` cannot be linted.

```
SublimeLinter: #1 __init__.py:1273    ERROR: Unhandled exception:
Traceback (most recent call last):
  File "C:\\Users\\Clover\\AppData\\Roaming\\Sublime Text 3\\Installed Packages\\SublimeLinter.sublime-package\\lint/backend.py", line 155, in execute_lint_task
    errors = linter.lint(code, view_has_changed)
  File "C:\\Users\\Clover\\AppData\\Roaming\\Sublime Text 3\\Installed Packages\\SublimeLinter.sublime-package\\lint/linter.py", line 1085, in lint
    output = self.run(cmd, code)
  File "C:\\Users\\Clover\\AppData\\Roaming\\Sublime Text 3\\Installed Packages\\SublimeLinter.sublime-package\\lint/linter.py", line 1469, in run
    return self.communicate(cmd, code)
  File "C:\\Users\\Clover\\AppData\\Roaming\\Sublime Text 3\\Installed Packages\\SublimeLinter.sublime-package\\lint/linter.py", line 1480, in communicate
    return self._communicate(cmd, code)
  File "C:\\Users\\Clover\\AppData\\Roaming\\Sublime Text 3\\Installed Packages\\SublimeLinter.sublime-package\\lint/linter.py", line 1576, in _communicate
    'Running ...', cmd, uses_stdin, cwd, view, env=None))
  File "C:\\Users\\Clover\\AppData\\Roaming\\Sublime Text 3\\Installed Packages\\SublimeLinter.sublime-package\\lint/linter.py", line 1668, in make_nice_log_message
    rel_filename = os.path.relpath(filename, cwd)
  File "./python3.3/ntpath.py", line 564, in relpath
ValueError: path is on mount 'C:', start on mount 'E:'
SublimeLinter: sublime_linter.py:482  Linting 'test.cpp' took 0.05s
```

## Related?

https://github.com/SublimeLinter/SublimeLinter/issues/1179
https://github.com/SublimeLinter/SublimeLinter/issues/721
https://github.com/SublimeLinter/SublimeLinter-gcc/pull/17#issuecomment-514397950
diff --git a/lint/linter.py b/lint/linter.py
index 1d129a1..28c6570 100644
--- a/lint/linter.py
+++ b/lint/linter.py
@@ -1677,7 +1677,11 @@ def make_nice_log_message(headline, cmd, is_stdin,
 
     filename = view.file_name()
     if filename and cwd:
-        rel_filename = os.path.relpath(filename, cwd)
+        rel_filename = (
+            os.path.relpath(filename, cwd)
+            if os.path.commonprefix([filename, cwd])
+            else filename
+        )
     elif not filename:
         rel_filename = '<buffer {}>'.format(view.buffer_id())
 
['lint/linter.py']