completion: fix completion of git <TAB><TAB>

completion: fix completion of git <TAB><TAB>

After commit 511a3fc (wrap git's main usage string., 2009-09-12), the
bash completion for git commands includes COMMAND and [ARGS] when it
shouldn't. Fix this by grepping more strictly for a line with git
commands. It's doubtful whether git will ever have commands starting
with anything besides numbers and letters so this should be fine. At
least by being stricter we'll know when we break the completion earlier.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>