submodule add: If --branch is given, record it in .gitmodules

This allows you to easily record a submodule.<name>.branch option in
.gitmodules when you add a new submodule.  With this patch,

  $ git submodule add -b <branch> <repository> [<path>]
  $ git config -f .gitmodules submodule.<path>.branch <branch>

reduces to

  $ git submodule add -b <branch> <repository> [<path>]

This means that future calls to

  $ git submodule update --remote ...

will get updates from the same branch that you used to initialize the
submodule, which is usually what you want.

Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -208,6 +208,8 @@ OPTIONS
 -b::
 --branch::
 	Branch of repository to add as submodule.
+	The name of the branch is recorded as `submodule.<path>.branch` in
+	`.gitmodules` for `update --remote`.
 
 -f::
 --force::

git-submodule.sh

@@ -417,6 +417,10 @@ Use -f if you really want to add it." >&2
 
 	git config -f .gitmodules submodule."$sm_name".path "$sm_path" &&
 	git config -f .gitmodules submodule."$sm_name".url "$repo" &&
+	if test -n "$branch"
+	then
+		git config -f .gitmodules submodule."$sm_name".branch "$branch"
+	fi &&
 	git add --force .gitmodules ||
 	die "$(eval_gettext "Failed to register submodule '\$sm_path'")"
 }

t/t7400-submodule-basic.sh

@@ -133,6 +133,7 @@ test_expect_success 'submodule add --branch' '
 	(
 		cd addtest &&
 		git submodule add -b initial "$submodurl" submod-branch &&
+		test "initial" = "$(git config -f .gitmodules submodule.submod-branch.branch)" &&
 		git submodule init
 	) &&