fetch-pack: do not remove .git/shallow file when --depth is not specified

fetch_pack() can remove .git/shallow file when a shallow repository
becomes a full one again. This behavior is triggered incorrectly when
tags are also fetched because fetch_pack() will be called twice. At
the first fetch_pack() call:

 - shallow_lock is set up
 - alternate_shallow_file points to shallow_lock.filename, which is
   "shallow.lock"
 - commit_lock_file is called, which sets shallow_lock.filename to "".
   alternate_shallow_file also becomes "" because it points to the
   same memory.

At the second call, setup_alternate_shallow() is not called and
alternate_shallow_file remains "". It's mistaken as unshallow case and
.git/shallow is removed. The end result is a broken repository.

Fix this by always initializing alternate_shallow_file when
fetch_pack() is called. As an extra measure, check if args->depth > 0
before commit/rollback shallow file.

Reported-by: Kacper Kornet <kornet@camk.edu.pl>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fetch-pack.c

@@ -888,6 +888,8 @@ static struct ref *do_fetch_pack(struct fetch_pack_args *args,
 		packet_flush(fd[1]);
 	if (args->depth > 0)
 		setup_alternate_shallow();
+	else
+		alternate_shallow_file = NULL;
 	if (get_pack(args, fd, pack_lockfile))
 		die("git fetch-pack: fetch failed.");
 
@@ -978,7 +980,7 @@ struct ref *fetch_pack(struct fetch_pack_args *args,
 	}
 	ref_cpy = do_fetch_pack(args, fd, ref, sought, nr_sought, pack_lockfile);
 
-	if (alternate_shallow_file) {
+	if (args->depth > 0 && alternate_shallow_file) {
 		if (*alternate_shallow_file == '\0') { /* --unshallow */
 			unlink_or_warn(git_path("shallow"));
 			rollback_lock_file(&shallow_lock);

t/t5500-fetch-pack.sh

@@ -505,4 +505,20 @@ test_expect_success 'test --all, --depth, and explicit tag' '
 	) >out-adt 2>error-adt
 '
 
+test_expect_success 'shallow fetch with tags does not break the repository' '
+	mkdir repo1 &&
+	(
+		cd repo1 &&
+		git init &&
+		test_commit 1 &&
+		test_commit 2 &&
+		test_commit 3 &&
+		mkdir repo2 &&
+		cd repo2 &&
+		git init &&
+		git fetch --depth=2 ../.git master:branch &&
+		git fsck
+	)
+'
+
 test_done