git-gui: Show all fetched branches for remote pulls.

Loop through every remote.<name>.fetch entry and add it as a valid
option in the Pull menu.  This way users can pull any remote branch
that they track, without needing to leave the gui.  Its a rather crude
work around for not having a full merge interface.

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