So far libgit2 didn't handle the following scenarios:

* Rename of reference m   -> m/m
* Rename of reference n/n -> n

Fixed.

Since we don't write reflogs, we have to delete any old reflog for the
renamed reference. Otherwise git.git will possibly fail when it finds
invalid logs.

Reported-by: nulltoken <emeric.fermas@gmail.com>
Signed-off-by: schu <schu-github@schulog.org>