Git: Allow direct push after commit

Git commit dialog: Added a drop-down menu to the commit button to
execute "git push" or open the "Push to Gerrit" dialog directly after
a commit or amend.

Task-number: QTCREATORBUG-8854
Change-Id: I00ff8f816d1768e0cdaf6929126b55826788e578
Reviewed-by: Petar Perisin <petar.perisin@gmail.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>