Current implementation of git_reference_rename() removes 'ref' from
loose cache, but not frees it. In result 'ref' is not reachable any more
and we have got memory leak.

Let's re-add 'ref' with corrected name to loose cache instead of
'new_ref' and free 'new_ref' properly.

'rollback' path seems leak too. git_reference_rename() need to be rewritten
for proper resource management.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>