git-gui: use +/- instead of ]/[ to show more/less context in diff

git-gui: use +/- instead of ]/[ to show more/less context in diff

On some systems, brackets cannot be used as event details
(they don't have a keysym), so use +/- instead (both on
keyboard and keypad) and add ctrl-= as a synonym of ctrl-+
for convenience.

[sp: Had to change accelerator to show only "$M1T-="; the
     original version included "$M1T-+ $M1T-=" but this is
	 not drawn at all on Mac OS X.]

Signed-off-by: Michele Ballabio <redacted>
Signed-off-by: Shawn O. Pearce <redacted>