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

revision.c

@@ -3111,12 +3111,12 @@ static void track_linear(struct rev_info *revs, struct commit *commit)
 
 static struct commit *get_revision_1(struct rev_info *revs)
 {
-	if (!revs->commits)
-		return NULL;
-
-	do {
+	while (1) {
 		struct commit *commit = pop_commit(&revs->commits);
 
+		if (!commit)
+			return NULL;
+
 		if (revs->reflog_info) {
 			save_parents(revs, commit);
 			fake_reflog_parent(revs->reflog_info, commit);
@@ -3150,8 +3150,7 @@ static struct commit *get_revision_1(struct rev_info *revs)
 				track_linear(revs, commit);
 			return commit;
 		}
-	} while (revs->commits);
-	return NULL;
+	}
 }
 
 /*