checkout: clear commit marks after detached-orphan check

When leaving a detached HEAD, we do a revision walk to make
sure the commit we are leaving isn't being orphaned.
However, this leaves crufty marks in the commit objects
which can confuse later walkers, like the one in
stat_tracking_info.

Let's clean up after ourselves to prevent this conflict.

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

builtin/checkout.c

@@ -603,6 +603,16 @@ static int add_one_ref_to_rev_list_arg(const char *refname,
 	return 0;
 }
 
+static int clear_commit_marks_from_one_ref(const char *refname,
+				      const unsigned char *sha1,
+				      int flags,
+				      void *cb_data)
+{
+	struct commit *commit = lookup_commit_reference_gently(sha1, 1);
+	if (commit)
+		clear_commit_marks(commit, -1);
+	return 0;
+}
 
 static void describe_one_orphan(struct strbuf *sb, struct commit *commit)
 {
@@ -674,6 +684,9 @@ static void orphaned_commit_warning(struct commit *commit)
 		suggest_reattach(commit, &revs);
 	else
 		describe_detached_head("Previous HEAD position was", commit);
+
+	clear_commit_marks(commit, -1);
+	for_each_ref(clear_commit_marks_from_one_ref, NULL);
 }
 
 static int switch_branches(struct checkout_opts *opts, struct branch_info *new)

t/t2020-checkout-detach.sh

@@ -126,4 +126,17 @@ test_expect_success 'checkout does not warn leaving reachable commit' '
 	check_no_orphan_warning stderr
 '
 
+cat >expect <<'EOF'
+Your branch is behind 'master' by 1 commit, and can be fast-forwarded.
+EOF
+test_expect_success 'tracking count is accurate after orphan check' '
+	reset &&
+	git branch child master^ &&
+	git config branch.child.remote . &&
+	git config branch.child.merge refs/heads/master &&
+	git checkout child^ &&
+	git checkout child >stdout &&
+	test_cmp expect stdout
+'
+
 test_done