Merge branch 'cp/submodule-custom-update'

In addition to the choice from "rebase, merge, or checkout-detach",
allow a custom command to be used in "submodule update" to update
the working tree of submodules.

* cp/submodule-custom-update:
  submodule update: allow custom command to update submodule working tree

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

git-submodule.sh

@@ -860,6 +860,12 @@ Maybe you want to use 'update --init'?")"
 				say_msg="$(eval_gettext "Submodule path '\$displaypath': merged in '\$sha1'")"
 				must_die_on_failure=yes
 				;;
+			!*)
+				command="${update_module#!}"
+				die_msg="$(eval_gettext "Execution of '\$command \$sha1' failed in submodule  path '\$prefix\$sm_path'")"
+				say_msg="$(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 '\$displaypath'")"

t/t7406-submodule-update.sh

@@ -294,6 +294,35 @@ test_expect_success 'submodule update - checkout in .git/config' '
 	)
 '
 
+test_expect_success 'submodule update - command in .git/config' '
+	(cd super &&
+	 git config submodule.submodule.update "!git checkout"
+	) &&
+	(cd super/submodule &&
+	  git reset --hard HEAD^
+	) &&
+	(cd super &&
+	 (cd submodule &&
+	  compare_head
+	 ) &&
+	 git submodule update submodule &&
+	 cd submodule &&
+	 ! compare_head
+	)
+'
+
+test_expect_success 'submodule update - command in .git/config catches failure' '
+	(cd super &&
+	 git config submodule.submodule.update "!false"
+	) &&
+	(cd super/submodule &&
+	  git reset --hard HEAD^
+	) &&
+	(cd super &&
+	 test_must_fail git submodule update submodule
+	)
+'
+
 test_expect_success 'submodule init picks up rebase' '
 	(cd super &&
 	 git config -f .gitmodules submodule.rebasing.update rebase &&