completion: new function __git_complete_log_opts
The options accepted by git-log are also accepted by at least one other
command (git-bisect). Factor the common option completion code into a
new function and use it from _git_log. The new function leaves
COMPREPLY empty if no option candidates are found, so that callers can
safely check it to determine if completion for other arguments should be
attempted.
Signed-off-by: Britton Leo Kerin <britton.kerin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>