bash: use for-each-ref format 'refname:short'

bash: use for-each-ref format 'refname:short'

Using this format simplifies the code for completing refs and (in some
cases) improves performance significantly.

For repositories like the current git.git (with more than 200 refs)
there is no real performance difference, but for a repository with 2000
refs the total time needed to complete the refs is reduced by ~25% (from
around 400ms to around 305ms).

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>