git-gui: Makefile: consolidate .FORCE-* targets

Providing multiple targets to force a rebuild is unnecessary
complication.

Avoid using a name that could conflict with future special
targets in GNU make (a leading period followed by uppercase
letters).

Cc: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>