gui: git cola can now rename a branch

Add support for renaming a branch from the menu.

Requested-by: Ｖ字龍(Vdragon) <Vdragon.Taiwan@gmail.com>
Signed-off-by: Minarto Margoliono <lie.r.min.g@gmail.com>
Merge pull request #364 from mmargoliono/rename-branch

gui: git cola can now rename a branch

Signed-off-by: David Aguilar <davvid@gmail.com>