merge-base: handle --fork-point without reflog

The --fork-point option looks in the reflog to try to find
where a derived branch forked from a base branch. However,
if the reflog for the base branch is totally empty (as it
commonly is right after cloning, which does not write a
reflog entry), then our for_each_reflog call will not find
any entries, and we will come up with no merge base, even
though there may be one with the current tip of the base.

We can fix this by just adding the current tip to
our list of collected entries.

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

builtin/merge-base.c

@@ -173,6 +173,9 @@ static int handle_fork_point(int argc, const char **argv)
 	revs.initial = 1;
 	for_each_reflog_ent(refname, collect_one_reflog_ent, &revs);
 
+	if (!revs.nr && !get_sha1(refname, sha1))
+		add_one_commit(sha1, &revs);
+
 	for (i = 0; i < revs.nr; i++)
 		revs.commit[i]->object.flags &= ~TMP_MARK;
 

t/t6010-merge-base.sh

@@ -258,6 +258,12 @@ test_expect_success 'using reflog to find the fork point' '
 	test_cmp expect3 actual
 '
 
+test_expect_success '--fork-point works with empty reflog' '
+	git -c core.logallrefupdates=false branch no-reflog base &&
+	git merge-base --fork-point no-reflog derived &&
+	test_cmp expect3 actual
+'
+
 test_expect_success 'merge-base --octopus --all for complex tree' '
 	# Best common ancestor for JE, JAA and JDD is JC
 	#             JE