Merge branch 'jk/checkout-from-tree' into maint

"git checkout $treeish $path", when $path in the index and the
working tree already matched what is in $treeish at the $path,
still overwrote the $path unnecessarily.

* jk/checkout-from-tree:
  checkout $tree: do not throw away unchanged index entries

builtin/checkout.c

@@ -67,6 +67,7 @@ static int update_some(const unsigned char *sha1, const char *base, int baselen,
 {
 	int len;
 	struct cache_entry *ce;
+	int pos;
 
 	if (S_ISDIR(mode))
 		return READ_TREE_RECURSIVE;
@@ -79,6 +80,23 @@ static int update_some(const unsigned char *sha1, const char *base, int baselen,
 	ce->ce_flags = create_ce_flags(0) | CE_UPDATE;
 	ce->ce_namelen = len;
 	ce->ce_mode = create_ce_mode(mode);
+
+	/*
+	 * If the entry is the same as the current index, we can leave the old
+	 * entry in place. Whether it is UPTODATE or not, checkout_entry will
+	 * do the right thing.
+	 */
+	pos = cache_name_pos(ce->name, ce->ce_namelen);
+	if (pos >= 0) {
+		struct cache_entry *old = active_cache[pos];
+		if (ce->ce_mode == old->ce_mode &&
+		    !hashcmp(ce->sha1, old->sha1)) {
+			old->ce_flags |= CE_UPDATE;
+			free(ce);
+			return 0;
+		}
+	}
+
 	add_cache_entry(ce, ADD_CACHE_OK_TO_ADD | ADD_CACHE_OK_TO_REPLACE);
 	return 0;
 }

t/t2022-checkout-paths.sh

@@ -61,4 +61,21 @@ test_expect_success 'do not touch unmerged entries matching $path but not in $tr
 	test_cmp expect.next0 actual.next0
 '
 
+test_expect_success 'do not touch files that are already up-to-date' '
+	git reset --hard &&
+	echo one >file1 &&
+	echo two >file2 &&
+	git add file1 file2 &&
+	git commit -m base &&
+	echo modified >file1 &&
+	test-chmtime =1000000000 file2 &&
+	git update-index -q --refresh &&
+	git checkout HEAD -- file1 file2 &&
+	echo one >expect &&
+	test_cmp expect file1 &&
+	echo "1000000000	file2" >expect &&
+	test-chmtime -v +0 file2 >actual &&
+	test_cmp expect actual
+'
+
 test_done