git-branch has an --abbrev= command line option, but it does
no checking of the input. Take the argument parsing code from
setup_revisions in revisions.c, and also the code for parsing
the --no-abbrev option.
Signed-off-by: Julian Phillips <julian@quantumfyre.co.uk> Signed-off-by: Junio C Hamano <junkio@cox.net>