Windows task conflict
As reported by @berezovskyi on IRC:

```
ERROR: 'output\\posts\\math-test-new\\index.html' is a target for render_indexes:output\\posts\\math-test-new\\index.html and render_pages:output\\posts\\math-test-new\\index.html
```

Happens if more than one post exists; Windows and v7.7.0 only.  Probably needs some `git bisect` first.

Repo: https://github.com/berezovskyi/nikola-troubleshoot

diff --git a/nikola/post.py b/nikola/post.py
index 94e1e7453..f9905e5a5 100644
--- a/nikola/post.py
+++ b/nikola/post.py
@@ -769,7 +769,7 @@ class Post(object):
 
         if not self.config['POSTS_SECTION_FROM_META']:
             dest = self.destination_path(lang)
-            if dest[-(1 + len(self.index_file)):] == '/' + self.index_file:
+            if dest[-(1 + len(self.index_file)):] == os.sep + self.index_file:
                 dest = dest[:-(1 + len(self.index_file))]
             dirname = os.path.dirname(dest)
             slug = dest.split(os.sep)
['nikola/post.py']