git-gui: Makefile: consolidate .FORCE-* targets
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 <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>
Signed-off-by: Jonathan Nieder <b57189c5f2fd5b3daf3350a77fde90def1080fa5@gmail.com>
Signed-off-by: Shawn O. Pearce <f01032651c12a22b2103346ba96b3b6b231f9d3c@spearce.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git