revision: do not fallback to default when rev_input_given is set

If revs->def is set (as it is in "git log") and there are no
pending objects after parsing the user's input, then we show
whatever is in "def". But if the user _did_ ask for some
input that just happened to be empty (e.g., "--glob" that
does not match anything), showing the default revision is
confusing. We should just show nothing, as that is what the
user's request yielded.

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

revision.c

@@ -2316,7 +2316,7 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 		opt->tweak(revs, opt);
 	if (revs->show_merge)
 		prepare_show_merge(revs);
-	if (revs->def && !revs->pending.nr && !got_rev_arg) {
+	if (revs->def && !revs->pending.nr && !revs->rev_input_given && !got_rev_arg) {
 		struct object_id oid;
 		struct object *object;
 		struct object_context oc;

t/t4202-log.sh

@@ -1523,6 +1523,12 @@ test_expect_success 'log diagnoses bogus HEAD' '
 	test_i18ngrep broken stderr
 '
 
+test_expect_success 'log does not default to HEAD when rev input is given' '
+	>expect &&
+	git log --branches=does-not-exist >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'set up --source tests' '
 	git checkout --orphan source-a &&
 	test_commit one &&