get_revision_1(): replace do-while with an early return

get_revision_1(): replace do-while with an early return

The get_revision_1() function tries to avoid entering its
main loop at all when there are no commits to look at. But
it's perfectly safe to call pop_commit() on an empty list
(in which case it will return NULL). Switching to an early
return from the loop lets us skip repeating the loop
condition before we enter the do-while. That will get more
important when we start pulling reflog-walk commits from a
source besides the revs->commits queue, as that condition
will get much more complicated.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>