This makes commands such as `git for-each-ref --format='%(refname)'`,
which are used heavily by the bash_completion code, run about 6 times
faster on an uncached repository (3 s intead of 18 s on my linux-2.6
repository with several remotes).
Signed-off-by: Anders Kaseorg <andersk@mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>