bash: Remove short option completions for branch/checkout/diff.

bash: Remove short option completions for branch/checkout/diff.

The short options (-l, -f, -d) for git-branch are rather silly to
include in the completion generation as these options must be fully
typed out by the user and most users already know what the options
are anyway, so including them in the suggested completions does
not offer huge value.  (The same goes for git-checkout and git-diff.)

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>