bisect: squelch "fatal: ref HEAD not a symref" misleading message

bisect: squelch "fatal: ref HEAD not a symref" misleading message

To get the current HEAD when we start bisecting using for example
"git bisect start", we first try "git symbolic-ref HEAD" to get a
nice name, and if it fails, we fall back to "git rev-parse
--verify HEAD".

The problem is that when "git symbolic-ref HEAD" fails, it
displays "fatal: ref HEAD not a symref", so it looks like "git
bisect start" failed and does not accept detached HEAD, even if
in fact it worked fine.

This patch adds "-q" option to the "git symbolic-ref" call to
get rid of the misleading error message.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>