git-gui: Allow users to match remote branch names locally

git-gui: Allow users to match remote branch names locally

Some workflows have users create a local branch that matches a remote
branch they have fetched from another repository.  If the user wants
to push their changes back to that remote repository then they probably
want to use the same branch name locally so that git-gui's push dialog
can setup the push refspec automatically.

To prevent typos with the local branch name we now offer an option to
use the remote tracking branch name as the new local branch name.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>