Git: Make branch lookup more robust

It might fail because a particular branch points to an invalid commit.
We don't want that to prevent display of all other branches.

Change-Id: I8fe427735351fc458c99396dc1f9d77bc948468e
Reviewed-by: Petar Perisin <petar.perisin@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>