Retry renaming files on Access Denied errors

When a file is open for reading (without shared-delete permission), and
then a different thread/process called p_rename, that would fail, even
if the file was only open for reading for a few milliseconds. This
change lets p_rename wait up to 50ms for the file to be closed by the
reader. Applies only to win32.

This is especially important for git_filebuf_commit, because writes
should not fail if the file is read simultaneously.

Fixes #2207