Merge branch 'fp/submodule-checkout-mode'

"submodule.*.update=checkout", when propagated from .gitmodules to
.git/config, turned into a "submodule.*.update=none", which did not
make much sense.

* fp/submodule-checkout-mode:
  git-submodule.sh: 'checkout' is a valid update mode

git-submodule.sh

@@ -622,7 +622,7 @@ cmd_init()
 		   test -z "$(git config submodule."$name".update)"
 		then
 			case "$upd" in
-			rebase | merge | none)
+			checkout | rebase | merge | none)
 				;; # known modes of updating
 			*)
 				echo >&2 "warning: unknown update mode '$upd' suggested for submodule '$name'"
@@ -805,6 +805,17 @@ cmd_update()
 			update_module=$update
 		else
 			update_module=$(git config submodule."$name".update)
+			case "$update_module" in
+			'')
+				;; # Unset update mode
+			checkout | rebase | merge | none)
+				;; # Known update modes
+			!*)
+				;; # Custom update command
+			*)
+				die "$(eval_gettext "Invalid update mode '$update_module' for submodule '$name'")"
+				;;
+			esac
 		fi
 
 		displaypath=$(relative_path "$prefix$sm_path")