git-gui: Include a Push action on the left toolbar

Pushing changes to a remote system is a very common action for
many users of git-gui, so much so that in some workflows a user
is supposed to push immediately after they make a local commit
so that their change(s) are immediately available for their
teammates to view and build on top of.

Including the push button right below the commit button on the
left toolbar indicates that users should probably perform this
action after they have performed the commit action.

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

git-gui.sh

@@ -1822,6 +1822,10 @@ pack .vpane.lower.commarea.buttons.commit -side top -fill x
 lappend disable_on_lock \
 	{.vpane.lower.commarea.buttons.commit conf -state}
 
+button .vpane.lower.commarea.buttons.push -text {Push} \
+	-command do_push_anywhere
+pack .vpane.lower.commarea.buttons.push -side top -fill x
+
 # -- Commit Message Buffer
 #
 frame .vpane.lower.commarea.buffer