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>

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