submodule: fall back to remote's HEAD for missing remote.<name>.branch

When `remote.<name>.branch` is not configured, `git submodule update`
currently falls back to using the branch name `master`. A much better
idea, however, is to use the remote `HEAD`: on all Git servers running
reasonably recent Git versions, the symref `HEAD` points to the main
branch.

Note: t7419 demonstrates that there _might_ be use cases out there that
_expect_ `git submodule update --remote` to update submodules to the
remote `master` branch even if the remote `HEAD` points to another
branch. Arguably, this patch makes the behavior more intuitive, but
there is a slight possibility that this might cause regressions in
obscure setups.

Even so, it should be okay to fix this behavior without anything like a
longer transition period:

- The `git submodule update --remote` command is not really common.

- Current Git's behavior when running this command is outright
  confusing, unless the remote repository's current branch _is_ `master`
  (in which case the proposed behavior matches the old behavior).

- If a user encounters a regression due to the changed behavior, the fix
  is actually trivial: setting `submodule.<name>.branch` to `master`
  will reinstate the old behavior.

Helped-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -183,7 +183,7 @@ set-branch (-d|--default) [--] <path>::
 	Sets the default remote tracking branch for the submodule. The
 	`--branch` option allows the remote branch to be specified. The
 	`--default` option removes the submodule.<name>.branch configuration
-	key, which causes the tracking branch to default to 'master'.
+	key, which causes the tracking branch to default to the remote 'HEAD'.
 
 set-url [--] <path> <newurl>::
 	Sets the URL of the specified submodule to <newurl>. Then, it will
@@ -284,7 +284,7 @@ OPTIONS
 	`.gitmodules` for `update --remote`.  A special value of `.` is used to
 	indicate that the name of the branch in the submodule should be the
 	same name as the current branch in the current repository.  If the
-	option is not specified, it defaults to 'master'.
+	option is not specified, it defaults to the remote 'HEAD'.
 
 -f::
 --force::
@@ -322,10 +322,10 @@ OPTIONS
 	the superproject's recorded SHA-1 to update the submodule, use the
 	status of the submodule's remote-tracking branch.  The remote used
 	is branch's remote (`branch.<name>.remote`), defaulting to `origin`.
-	The remote branch used defaults to `master`, but the branch name may
+	The remote branch used defaults to the remote `HEAD`, but the branch
-	be overridden by setting the `submodule.<name>.branch` option in
+	name may be overridden by setting the `submodule.<name>.branch`
-	either `.gitmodules` or `.git/config` (with `.git/config` taking
+	option in either `.gitmodules` or `.git/config` (with `.git/config`
-	precedence).
+	taking precedence).
 +
 This works for any of the supported update procedures (`--checkout`,
 `--rebase`, etc.).  The only change is the source of the target SHA-1.

Documentation/gitmodules.txt

@@ -49,9 +49,9 @@ submodule.<name>.update::
 
 submodule.<name>.branch::
 	A remote branch name for tracking updates in the upstream submodule.
-	If the option is not specified, it defaults to 'master'.  A special
+	If the option is not specified, it defaults to the remote 'HEAD'.
-	value of `.` is used to indicate that the name of the branch in the
+	A special value of `.` is used to indicate that the name of the branch
-	submodule should be the same name as the current branch in the
+	in the submodule should be the same name as the current branch in the
 	current repository.  See the `--remote` documentation in
 	linkgit:git-submodule[1] for details.
 

builtin/submodule--helper.c

@@ -1981,7 +1981,7 @@ static const char *remote_submodule_branch(const char *path)
 	free(key);
 
 	if (!branch)
-		return "master";
+		return "HEAD";
 
 	if (!strcmp(branch, ".")) {
 		const char *refname = resolve_ref_unsafe("HEAD", 0, NULL, NULL);

t/t7406-submodule-update.sh

@@ -70,6 +70,22 @@ test_expect_success 'setup a submodule tree' '
 	)
 '
 
+test_expect_success 'update --remote falls back to using HEAD' '
+	test_create_repo main-branch-submodule &&
+	test_commit -C main-branch-submodule initial &&
+
+	test_create_repo main-branch &&
+	git -C main-branch submodule add ../main-branch-submodule &&
+	git -C main-branch commit -m add-submodule &&
+
+	git -C main-branch-submodule switch -c hello &&
+	test_commit -C main-branch-submodule world &&
+
+	git clone --recursive main-branch main-branch-clone &&
+	git -C main-branch-clone submodule update --remote main-branch-submodule &&
+	test_path_exists main-branch-clone/main-branch-submodule/world.t
+'
+
 test_expect_success 'submodule update detaching the HEAD ' '
 	(cd super/submodule &&
 	 git reset --hard HEAD~1

t/t7419-submodule-set-branch.sh

@@ -52,12 +52,13 @@ test_expect_success 'test submodule set-branch --branch' '
 '
 
 test_expect_success 'test submodule set-branch --default' '
+	test_commit -C submodule c &&
 	(cd super &&
 		git submodule set-branch --default submodule &&
 		! grep branch .gitmodules &&
 		git submodule update --remote &&
 		cat <<-\EOF >expect &&
-		a
+		c
 		EOF
 		git -C submodule show -s --pretty=%s >actual &&
 		test_cmp expect actual
@@ -65,6 +66,7 @@ test_expect_success 'test submodule set-branch --default' '
 '
 
 test_expect_success 'test submodule set-branch -b' '
+	test_commit -C submodule b &&
 	(cd super &&
 		git submodule set-branch -b topic submodule &&
 		grep "branch = topic" .gitmodules &&
@@ -78,12 +80,13 @@ test_expect_success 'test submodule set-branch -b' '
 '
 
 test_expect_success 'test submodule set-branch -d' '
+	test_commit -C submodule d &&
 	(cd super &&
 		git submodule set-branch -d submodule &&
 		! grep branch .gitmodules &&
 		git submodule update --remote &&
 		cat <<-\EOF >expect &&
-		a
+		d
 		EOF
 		git -C submodule show -s --pretty=%s >actual &&
 		test_cmp expect actual