git-gui: Modify push dialog to support Gerrit review

Problem: It is not possible to push for Gerrit review as you will
always try to push to "refs/heads/..." on the remote.

Changes done:

Add an option to select "Gerrit review" and a corresponding entry
for a branch name. If this option is selected, push the changes to
"refs/for/<gerrit-branch>/<local branch>". In this way the local branch
names will be used as topic branches on Gerrit.