[RFC/PATCHv3] submodule update: allow custom update command

[Chris Packham <judge.packham@gmail.com>]

Users can set submodule.$name.update to '!command' which will cause
'command' to be run instead of checkout/merge/rebase.  This allows the
user some finer grained control over how the update is done. The primary
motivation for this was interoperability with stgit however being able
to intercept the submodule update process may prove useful for
integrating or extending other tools.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
---
v3 updated as per Junio's review.

Still needs tests. Any suggestions? I've been manually testing by setting
submodule.$name.update to '!echo'. I haven't looked to see if there are
existing 'submodule update' tests yet.

 Documentation/git-submodule.txt | 5 ++++-
 git-submodule.sh                | 6 ++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/Documentation/git-submodule.txt b/Documentation/git-submodule.txt
index e576713..0befc20 100644
--- a/Documentation/git-submodule.txt
+++ b/Documentation/git-submodule.txt
@@ -159,7 +159,9 @@ update::
 	This will make the submodules HEAD be detached unless `--rebase` or
 	`--merge` is specified or the key `submodule.$name.update` is set to
 	`rebase`, `merge` or `none`. `none` can be overridden by specifying
-	`--checkout`.
+	`--checkout`. Setting the key `submodule.$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.
 +
 If the submodule is not yet initialized, and you just want to use the
 setting as stored in .gitmodules, you can automatically initialize the
@@ -172,6 +174,7 @@ If `--force` is specified, the submodule will be checked out (using
 `git checkout --force` if appropriate), even if the commit specified in the
 index of the containing repository already matches the commit checked out in
 the submodule.
++
 
 summary::
 	Show commit summary between the given commit (defaults to HEAD) and
diff --git a/git-submodule.sh b/git-submodule.sh
index eb58c8e..a7c2375 100755
--- a/git-submodule.sh
+++ b/git-submodule.sh
@@ -799,6 +799,12 @@ Maybe you want to use 'update --init'?")"
 				say_msg="$(eval_gettext "Submodule path '\$prefix\$sm_path': merged in '\$sha1'")"
 				must_die_on_failure=yes
 				;;
+			!*)
+				command="${update_module#!}"
+				die_msg="$(eval_gettext "Unable to exec '\$command \$sha1' in submodule path '\$prefix\$sm_path'")"
+				say_msg="$(eval_gettext "Submodule path '\$prefix\$sm_path': '\$command \$sha1'")"
+				must_die_on_failure=yes
+				;;
 			*)
 				command="git checkout $subforce -q"
 				die_msg="$(eval_gettext "Unable to checkout '\$sha1' in submodule path '\$prefix\$sm_path'")"
-- 
1.8.3.1.644.gfaf2af3