Merge branch 'jk/merge-base-fork-point-without-reflog' into maint

"git rebase" immediately after "git clone" failed to find the fork
point from the upstream.

* jk/merge-base-fork-point-without-reflog:
  merge-base: handle --fork-point without reflog

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

@@ -260,6 +260,12 @@ 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