submodule--helper: understand --checkout, --merge and --rebase synonyms

Understand --checkout, --merge and --rebase synonyms for
--update={checkout,merge,rebase}, as well as the short options that
'git submodule' itself understands.

This removes a difference between the CLI API of "git submodule" and
"git submodule--helper", making it easier to make the latter an alias
for the former. See 48308681b0 (git submodule update: have a
dedicated helper for cloning, 2016-02-29) for the initial addition of
--update.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -40,7 +40,9 @@ require_init=
 files=
 remote=
 nofetch=
-update=
+rebase=
+merge=
+checkout=
 custom_name=
 depth=
 progress=
@@ -260,7 +262,7 @@ cmd_update()
 			force=$1
 			;;
 		-r|--rebase)
-			update="rebase"
+			rebase=1
 			;;
 		--reference)
 			case "$2" in '') usage ;; esac
@@ -274,13 +276,13 @@ cmd_update()
 			dissociate=1
 			;;
 		-m|--merge)
-			update="merge"
+			merge=1
 			;;
 		--recursive)
 			recursive=1
 			;;
 		--checkout)
-			update="checkout"
+			checkout=1
 			;;
 		--recommend-shallow)
 			recommend_shallow="--recommend-shallow"
@@ -341,7 +343,9 @@ cmd_update()
 		${init:+--init} \
 		${nofetch:+--no-fetch} \
 		${wt_prefix:+--prefix "$wt_prefix"} \
-		${update:+--update "$update"} \
+		${rebase:+--rebase} \
+		${merge:+--merge} \
+		${checkout:+--checkout} \
 		${reference:+"$reference"} \
 		${dissociate:+"--dissociate"} \
 		${depth:+"$depth"} \