infinitepush: ensure fileindex bookmarks use '/' separators (issue5840)

After loading up with status messages, I noticed that the subsequent matcher was
rejecting 'scratch\mybranch' on Windows.  No bookmarks were reported back, and
the tests subsequently failed.  I did a search for 'match', and nothing else
looks like it needs to be fixed up, but someone who understands this code should
also take a look.

I also tried setting `infinitepush.branchpattern=re:scratch\\.*` in
library-infinitepush.sh without this change, but that didn't work.  Still,
should we ban '\' in these bookmarks to avoid confusion?  I thought I saw code
that sandwiches a pattern between 're:^' and '.*', so perhaps regex characters
will need special care?

I also noticed comments in externalbundlestore.{read,write} that it won't work
on Windows because of opening an open file.  But I don't see a test failure, so
this may lack test coverage.

hgext/infinitepush/fileindexapi.py

@@ -15,6 +15,8 b' from __future__ import absolute_import'
 
 import os
 
+from mercurial import util
+
 from mercurial.utils import stringutil
 
 from . import indexapi
@@ -82,6 +84,7 b' class fileindexapi(indexapi.indexapi):'
         for dirpath, _, books in self._repo.vfs.walk(self._bookmarkmap):
             for book in books:
                 bookmark = os.path.join(dirpath, book)[prefixlen:]
+                bookmark = util.pconvert(bookmark)
                 if not matcher(bookmark):
                     continue
                 yield bookmark, self._read(os.path.join(dirpath, book))