submodule: explicit local branch creation in module_clone

The previous code only checked out branches in cmd_add.  This commit
moves the branch-checkout logic into module_clone, where it can be
shared by cmd_add and cmd_update.  I also update the initial checkout
command to use 'reset' to preserve branches setup during module_clone.

With this change, folks cloning submodules for the first time via:

  $ git submodule update ...

will get a local branch instead of a detached HEAD, unless they are
using the default checkout-mode updates.  This is a change from the
previous situation where cmd_update always used checkout-mode logic
(regardless of the requested update mode) for updates that triggered
an initial clone, which always resulted in a detached HEAD.

This commit does not change the logic for updates after the initial
clone, which will continue to create detached HEADs for checkout-mode
updates, and integrate remote work with the local HEAD (detached or
not) in other modes.

The motivation for the change is that developers doing local work
inside the submodule are likely to select a non-checkout-mode for
updates so their local work is integrated with upstream work.
Developers who are not doing local submodule work stick with
checkout-mode updates so any apparently local work is blown away
during updates.  For example, if upstream rolls back the remote branch
or gitlinked commit to an earlier version, the checkout-mode developer
wants their old submodule checkout to be rolled back as well, instead
of getting a no-op merge/rebase with the rolled-back reference.

By using the update mode to distinguish submodule developers from
black-box submodule consumers, we can setup local branches for the
developers who will want local branches, and stick with detached HEADs
for the developers that don't care.

Testing
=======

In t7406, just-cloned checkouts now update to the gitlinked hash with
'reset', to preserve the local branch for situations where we're not
on a detached HEAD.

I also added explicit tests to t7406 for HEAD attachement after
cloning updates, showing that it depends on their update mode:

* Checkout-mode updates get detached HEADs
* Everyone else gets a local branch, matching the configured
  submodule.<name>.branch and defaulting to master.

The 'initial-setup' tag makes it easy to reset the superproject to a
known state, as several earlier tests commit to submodules and commit
the changed gitlinks to the superproject, but don't push the new
submodule commits to the upstream subprojects.  This makes it
impossible to checkout the current super master, because it references
submodule commits that don't exist in the upstream subprojects.  For a
specific example, see the tests that currently generate the
'two_new_submodule_commits' commits.

Documentation
=============

I updated the docs to describe the 'submodule update' modes in detail.
The old documentation did not distinguish between cloning and
non-cloning updates and lacked clarity on which operations would lead
to detached HEADs, and which would not.  The new documentation
addresses these issues while updating the docs to reflect the changes
introduced by this commit's explicit local branch creation in
module_clone.

I also add '--checkout' to the usage summary and group the update-mode
options into a single set.

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

1 files changed, 27 insertions, 9 deletions

diff --git a/Documentation/git-submodule.txt b/Documentation/git-submodule.txt
index bfef8a0c62..2e1c7a28d0 100644
--- a/Documentation/git-submodule.txt
+++ b/Documentation/git-submodule.txt
@@ -15,8 +15,8 @@ SYNOPSIS
 'git submodule' [--quiet] init [--] [<path>...]
 'git submodule' [--quiet] deinit [-f|--force] [--] <path>...
 'git submodule' [--quiet] update [--init] [--remote] [-N|--no-fetch]
-	      [-f|--force] [--rebase] [--reference <repository>] [--depth <depth>]
-	      [--merge] [--recursive] [--] [<path>...]
+	      [-f|--force] [--rebase|--merge|--checkout] [--reference <repository>]
+	      [--depth <depth>] [--recursive] [--] [<path>...]
 'git submodule' [--quiet] summary [--cached|--files] [(-n|--summary-limit) <n>]
 	      [commit] [--] [<path>...]
 'git submodule' [--quiet] foreach [--recursive] <command>
@@ -155,13 +155,31 @@ it contains local modifications.
 
 update::
 	Update the registered submodules, i.e. clone missing submodules and
-	checkout the commit specified in the index of the containing repository.
-	This will make the submodules HEAD be detached unless `--rebase` or
-	`--merge` is specified or the key `submodule.$name.update` is set to
-	`rebase`, `merge` or `none`. `none` can be overridden by specifying
-	`--checkout`. Setting the key `submodule.$name.update` to `!command`
-	will cause `command` to be run. `command` can be any arbitrary shell
-	command that takes a single argument, namely the sha1 to update to.
+	checkout the commit specified in the index of the containing
+	repository.  The update mode defaults to `checkout`, but can be
+	configured with the `submodule.<name>.update` setting or the
+	`--rebase`, `--merge`, or `--checkout` options.
++
+For updates that clone missing submodules, checkout-mode updates will
+create submodules with detached HEADs; all other modes will create
+submodules with a local branch named after `submodule.<path>.branch`.
++
+For updates that do not clone missing submodules, the submodule's HEAD
+is only touched when the remote reference does not match the
+submodule's HEAD (for none-mode updates, the submodule is never
+touched).  The remote reference is usually the gitlinked commit from
+the superproject's tree, but with `--remote` it is the upstream
+subproject's `submodule.<name>.branch`.  This remote reference is
+integrated with the submodule's HEAD using the specified update mode.
+For checkout-mode updates, that will result in a detached HEAD.  For
+rebase- and merge-mode updates, the commit referenced by the
+submodule's HEAD may change, but the symbolic reference will remain
+unchanged (i.e. checked-out branches will still be checked-out
+branches, and detached HEADs will still be detached HEADs).  If none
+of the builtin modes fit your needs, set `submodule.<name>.update` to
+`!command` to configure a custom integration command.  `command` can
+be any arbitrary shell command that takes a single argument, namely
+the sha1 to update to.
 +
 If the submodule is not yet initialized, and you just want to use the
 setting as stored in .gitmodules, you can automatically initialize the