Git: Added ability to checkout and reset in GitEditor

Especially useful in reflog.

Change-Id: I1d63221f152ebc7e69934482785b433d016e4ead
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Petar Perisin <petar.perisin@gmail.com>