fetch: enable deepening/shortening shallow clones

A shallow repository can currently only be completely unshallowed, which
is caused by mark_local() only marking locally-existing objects as
wanted if the fetch depth is set to INT_MAX (GIT_FETCH_DEPTH_UNSHALLOW).
This prevents deepening the history of a shallow clone to an arbitrary
number of commits, which may be preferable over full unshallowing for
large repositories.

Enable deepening and shortening shallow clones by marking
locally-existing objects as wanted whenever the fetch depth is set to
any non-default value (either GIT_FETCH_DEPTH_UNSHALLOW or an arbitrary
positive integer).

src/libgit2/fetch.c

@@ -60,9 +60,11 @@ static int mark_local(git_remote *remote)
 
 	git_vector_foreach(&remote->refs, i, head) {
 		/* If we have the object, mark it so we don't ask for it.
-		   However if we are unshallowing, we need to ask for it
-		   even though the head exists locally. */
-		if (remote->nego.depth != INT_MAX && git_odb_exists(odb, &head->oid))
+		   However if we are unshallowing or changing history
+		   depth, we need to ask for it even though the head
+		   exists locally. */
+		if (remote->nego.depth == GIT_FETCH_DEPTH_FULL &&
+		    git_odb_exists(odb, &head->oid))
 			head->local = 1;
 		else
 			remote->need_pack = 1;

tests/libgit2/online/shallow.c

@@ -164,3 +164,102 @@ void test_online_shallow__unshallow(void)
 	git_revwalk_free(walk);
 	git_repository_free(repo);
 }
+
+void test_online_shallow__deepen_six(void)
+{
+	git_str path = GIT_STR_INIT;
+	git_repository *repo;
+	git_revwalk *walk;
+	git_clone_options clone_opts = GIT_CLONE_OPTIONS_INIT;
+	git_fetch_options fetch_opts = GIT_FETCH_OPTIONS_INIT;
+	git_remote *origin = NULL;
+	git_oid oid;
+	git_oid *roots;
+	size_t roots_len;
+	size_t num_commits = 0;
+	int error = 0;
+
+	clone_opts.fetch_opts.depth = 5;
+	clone_opts.remote_cb = remote_single_branch;
+
+	git_str_joinpath(&path, clar_sandbox_path(), "deepen_6");
+	cl_git_pass(git_clone(&repo, "https://github.com/libgit2/TestGitRepository", git_str_cstr(&path), &clone_opts));
+	cl_assert_equal_b(true, git_repository_is_shallow(repo));
+
+	fetch_opts.depth = 6;
+	cl_git_pass(git_remote_lookup(&origin, repo, "origin"));
+	cl_git_pass(git_remote_fetch(origin, NULL, &fetch_opts, NULL));
+	cl_assert_equal_b(true, git_repository_is_shallow(repo));
+
+	cl_git_pass(git_repository__shallow_roots(&roots, &roots_len, repo));
+	cl_assert_equal_i(4, roots_len);
+	cl_assert_equal_s("58be4659bb571194ed4562d04b359d26216f526e", git_oid_tostr_s(&roots[0]));
+	cl_assert_equal_s("d31f5a60d406e831d056b8ac2538d515100c2df2", git_oid_tostr_s(&roots[1]));
+	cl_assert_equal_s("6462e7d8024396b14d7651e2ec11e2bbf07a05c4", git_oid_tostr_s(&roots[2]));
+	cl_assert_equal_s("2c349335b7f797072cf729c4f3bb0914ecb6dec9", git_oid_tostr_s(&roots[3]));
+
+	git_revwalk_new(&walk, repo);
+	git_revwalk_push_head(walk);
+
+	while ((error = git_revwalk_next(&oid, walk)) == GIT_OK) {
+		num_commits++;
+	}
+
+	cl_assert_equal_i(num_commits, 17);
+	cl_assert_equal_i(error, GIT_ITEROVER);
+
+	git__free(roots);
+	git_remote_free(origin);
+	git_str_dispose(&path);
+	git_revwalk_free(walk);
+	git_repository_free(repo);
+}
+
+void test_online_shallow__shorten_four(void)
+{
+	git_str path = GIT_STR_INIT;
+	git_repository *repo;
+	git_revwalk *walk;
+	git_clone_options clone_opts = GIT_CLONE_OPTIONS_INIT;
+	git_fetch_options fetch_opts = GIT_FETCH_OPTIONS_INIT;
+	git_remote *origin = NULL;
+	git_oid oid;
+	git_oid *roots;
+	size_t roots_len;
+	size_t num_commits = 0;
+	int error = 0;
+
+	clone_opts.fetch_opts.depth = 5;
+	clone_opts.remote_cb = remote_single_branch;
+
+	git_str_joinpath(&path, clar_sandbox_path(), "shorten_4");
+	cl_git_pass(git_clone(&repo, "https://github.com/libgit2/TestGitRepository", git_str_cstr(&path), &clone_opts));
+	cl_assert_equal_b(true, git_repository_is_shallow(repo));
+
+	fetch_opts.depth = 4;
+	cl_git_pass(git_remote_lookup(&origin, repo, "origin"));
+	cl_git_pass(git_remote_fetch(origin, NULL, &fetch_opts, NULL));
+	cl_assert_equal_b(true, git_repository_is_shallow(repo));
+
+	cl_git_pass(git_repository__shallow_roots(&roots, &roots_len, repo));
+	cl_assert_equal_i(3, roots_len);
+	cl_assert_equal_s("d86a2aada2f5e7ccf6f11880bfb9ab404e8a8864", git_oid_tostr_s(&roots[0]));
+	cl_assert_equal_s("59706a11bde2b9899a278838ef20a97e8f8795d2", git_oid_tostr_s(&roots[1]));
+	cl_assert_equal_s("bab66b48f836ed950c99134ef666436fb07a09a0", git_oid_tostr_s(&roots[2]));
+
+	git_revwalk_new(&walk, repo);
+	git_revwalk_push_head(walk);
+
+	while ((error = git_revwalk_next(&oid, walk)) == GIT_OK) {
+		num_commits++;
+	}
+
+	cl_assert_equal_i(num_commits, 10);
+	cl_assert_equal_i(error, GIT_ITEROVER);
+
+	git__free(roots);
+	git_remote_free(origin);
+	git_str_dispose(&path);
+	git_revwalk_free(walk);
+	git_repository_free(repo);
+}