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>

2 files changed, 9 insertions, 9 deletions

diff --git a/Documentation/git-submodule.txt b/Documentation/git-submodule.txt
index c9ed2bf3d5..7e5f995f77 100644
--- a/Documentation/git-submodule.txt
+++ b/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
-	be overridden by setting the `submodule.<name>.branch` option in
-	either `.gitmodules` or `.git/config` (with `.git/config` taking
-	precedence).
+	The remote branch used defaults to the remote `HEAD`, but the branch
+	name may be overridden by setting the `submodule.<name>.branch`
+	option in either `.gitmodules` or `.git/config` (with `.git/config`
+	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.
diff --git a/Documentation/gitmodules.txt b/Documentation/gitmodules.txt
index 67275fd187..539b4e1997 100644
--- a/Documentation/gitmodules.txt
+++ b/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
-	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
+	If the option is not specified, it defaults to the remote 'HEAD'.
+	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.  See the `--remote` documentation in
 	linkgit:git-submodule[1] for details.