refs/files: use correct error type when lock exists

When fetching references into a repository, if a lock for a particular
reference exists, then `lock_raw_ref()` throws:

    - REF_TRANSACTION_ERROR_CASE_CONFLICT: when there is a conflict
    because the transaction contains conflicting references while being
    on a case-insensitive filesystem.

    - REF_TRANSACTION_ERROR_GENERIC: for all other errors.

The latter causes the entire set of batched updates to fail, even in
case sensitive filessystems.

Instead, return a 'REF_TRANSACTION_ERROR_CREATE_EXISTS' error. This
allows batched updates to reject the individual update which conflicts
with the existing file, while updating the rest of the references.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5510-fetch.sh

@@ -1546,6 +1546,32 @@ test_expect_success CASE_INSENSITIVE_FS,REFFILES 'existing references in a case
 	)
 '
 
+test_expect_success REFFILES 'existing reference lock in repo' '
+	test_when_finished rm -rf base repo &&
+	(
+		git init --ref-format=reftable base &&
+		cd base &&
+		echo >file update &&
+		git add . &&
+		git commit -m "updated" &&
+		git branch -M main &&
+
+		git update-ref refs/heads/foo @ &&
+		git update-ref refs/heads/branch @ &&
+		cd .. &&
+
+		git init --ref-format=files --bare repo &&
+		cd repo &&
+		git remote add origin ../base &&
+		touch refs/heads/foo.lock &&
+		test_must_fail git fetch -f origin "refs/heads/*:refs/heads/*" 2>err &&
+		test_grep "error: fetching ref refs/heads/foo failed: reference already exists" err &&
+		git rev-parse refs/heads/main >expect &&
+		git rev-parse refs/heads/branch >actual &&
+		test_cmp expect actual
+	)
+'
+
 . "$TEST_DIRECTORY"/lib-httpd.sh
 start_httpd