git branch: fix performance problem

git branch: fix performance problem

'git branch' looks at _all_ the refs, and verifies them.  Which means that
during cold-cache situations with a slow disk (and lots of tags, for
example) it can take several very annoying seconds (7.5s according to a
report by Carlos R.  Mafra).

This avoids most of it by simply doing the filtering before looking up
the commits, by using the "raw" version of for_each_ref.

Reported-by: Carlos R. Mafra <crmafra2@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>