refs: return conflict error when checking packed refs

The TRANSACTION_NAME_CONFLICT error code refers to a failure to create a
ref due to a name conflict with another ref. An example of this is a
directory/file conflict such as ref names A/B and A.

"git fetch" uses this error code to more accurately describe the error
by recommending to the user that they try running "git remote prune" to
remove any old refs that are deleted by the remote which would clear up
any directory/file conflicts.

This helpful error message is not displayed when the conflicted ref is
stored in packed refs. This change fixes this by ensuring error return
code consistency in `lock_raw_ref`.

Signed-off-by: Ivan Tse <ivan.tse1@gmail.com>

refs/files-backend.c

@@ -794,8 +794,10 @@ static int lock_raw_ref(struct files_ref_store *refs,
 		 */
 		if (refs_verify_refname_available(
 				    refs->packed_ref_store, refname,
-				    extras, NULL, err))
+				    extras, NULL, err)) {
+			ret = TRANSACTION_NAME_CONFLICT;
 			goto error_return;
+		}
 	}
 
 	ret = 0;

t/t5510-fetch.sh

@@ -1091,6 +1091,22 @@ test_expect_success 'branchname D/F conflict resolved by --prune' '
 	test_cmp expect actual
 '
 
+test_expect_success 'branchname D/F conflict rejected with targeted error message' '
+	git clone . df-conflict-error &&
+	git branch dir_conflict &&
+	(
+		cd df-conflict-error &&
+		git update-ref refs/remotes/origin/dir_conflict/file HEAD &&
+		test_must_fail git fetch 2>err &&
+		test_grep "error: some local refs could not be updated; try running" err &&
+		test_grep " ${SQ}git remote prune origin${SQ} to remove any old, conflicting branches" err &&
+		git pack-refs --all &&
+		test_must_fail git fetch 2>err-packed &&
+		test_grep "error: some local refs could not be updated; try running" err-packed &&
+		test_grep " ${SQ}git remote prune origin${SQ} to remove any old, conflicting branches" err-packed
+	)
+'
+
 test_expect_success 'fetching a one-level ref works' '
 	test_commit extra &&
 	git reset --hard HEAD^ &&