Merge branch 'wk/submodule-update-remote'

The beginning of 'integrate with the tip of the remote branch, not
the commit recorded in the superproject gitlink' support.

* wk/submodule-update-remote:
  submodule add: If --branch is given, record it in .gitmodules
  submodule update: add --remote for submodule's upstream changes
  submodule: add get_submodule_config helper funtion

Documentation/config.txt

@@ -2007,6 +2007,12 @@ submodule.<name>.update::
 	URL and other values found in the `.gitmodules` file.  See
 	linkgit:git-submodule[1] and linkgit:gitmodules[5] for details.
 
+submodule.<name>.branch::
+	The remote branch name for a submodule, used by `git submodule
+	update --remote`.  Set this option to override the value found in
+	the `.gitmodules` file.  See linkgit:git-submodule[1] and
+	linkgit:gitmodules[5] for details.
+
 submodule.<name>.fetchRecurseSubmodules::
 	This option can be used to control recursive fetching of this
 	submodule. It can be overridden by using the --[no-]recurse-submodules

Documentation/git-submodule.txt

@@ -13,7 +13,7 @@ SYNOPSIS
 	      [--reference <repository>] [--] <repository> [<path>]
 'git submodule' [--quiet] status [--cached] [--recursive] [--] [<path>...]
 'git submodule' [--quiet] init [--] [<path>...]
-'git submodule' [--quiet] update [--init] [-N|--no-fetch] [--rebase]
+'git submodule' [--quiet] update [--init] [--remote] [-N|--no-fetch] [--rebase]
 	      [--reference <repository>] [--merge] [--recursive] [--] [<path>...]
 'git submodule' [--quiet] summary [--cached|--files] [(-n|--summary-limit) <n>]
 	      [commit] [--] [<path>...]
@@ -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::
@@ -236,6 +238,27 @@ OPTIONS
 	(the default). This limit only applies to modified submodules. The
 	size is always limited to 1 for added/deleted/typechanged submodules.
 
+--remote::
+	This option is only valid for the update command.  Instead of using
+	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).
++
+This works for any of the supported update procedures (`--checkout`,
+`--rebase`, etc.).  The only change is the source of the target SHA-1.
+For example, `submodule update --remote --merge` will merge upstream
+submodule changes into the submodules, while `submodule update
+--merge` will merge superproject gitlink changes into the submodules.
++
+In order to ensure a current tracking branch state, `update --remote`
+fetches the submodule's remote repository before calculating the
+SHA-1.  If you don't want to fetch, you should use `submodule update
+--remote --no-fetch`.
+
 -N::
 --no-fetch::
 	This option is only valid for the update command.

Documentation/gitmodules.txt

@@ -49,6 +49,11 @@ submodule.<name>.update::
 	This config option is overridden if 'git submodule update' is given
 	the '--merge', '--rebase' or '--checkout' options.
 
+submodule.<name>.branch::
+	A remote branch name for tracking updates in the upstream submodule.
+	If the option is not specified, it defaults to 'master'.  See the
+	`--remote` documentation in linkgit:git-submodule[1] for details.
+
 submodule.<name>.fetchRecurseSubmodules::
 	This option can be used to control recursive fetching of this
 	submodule. If this option is also present in the submodules entry in