don't use default revision if a rev was specified

If a revision is specified, it happens not to have any commits, don't
use the default revision.  By doing so, surprising and undesired
behavior can happen, such as showing the reflog for HEAD when a branch
was specified.

[jc: squashed a test from René]

Signed-off-by: Dave Olszewski <cxreg@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.c

@@ -1334,7 +1334,7 @@ static void append_prune_data(const char ***prune_data, const char **av)
  */
 int setup_revisions(int argc, const char **argv, struct rev_info *revs, const char *def)
 {
-	int i, flags, left, seen_dashdash, read_from_stdin;
+	int i, flags, left, seen_dashdash, read_from_stdin, got_rev_arg = 0;
 	const char **prune_data = NULL;
 
 	/* First, search for "--" */
@@ -1460,6 +1460,8 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, const ch
 			append_prune_data(&prune_data, argv + i);
 			break;
 		}
+		else
+			got_rev_arg = 1;
 	}
 
 	if (prune_data)
@@ -1469,7 +1471,7 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, const ch
 		revs->def = def;
 	if (revs->show_merge)
 		prepare_show_merge(revs);
-	if (revs->def && !revs->pending.nr) {
+	if (revs->def && !revs->pending.nr && !got_rev_arg) {
 		unsigned char sha1[20];
 		struct object *object;
 		unsigned mode;

t/t1411-reflog-show.sh

@@ -64,4 +64,13 @@ test_expect_success 'using --date= shows reflog date (oneline)' '
 	test_cmp expect actual
 '
 
+: >expect
+test_expect_success 'empty reflog file' '
+	git branch empty &&
+	: >.git/logs/refs/heads/empty &&
+
+	git log -g empty >actual &&
+	test_cmp expect actual
+'
+
 test_done