Ask git to checkout a submodule even if it is skipped in .gitmodules

Setting a submodule as `update = none` is a common way to make cargo
skip the submodule. Unfortunately this makes `actions/checkout` skip
the submodule too, even if it's configured to fetch submodules.

This patch adds an option that is a default in git but causes the
submodules to be available even if skipped otherwise.

See: #915
See: rust-lang/cargo#4247 (comment)
See: https://git-scm.com/docs/git-submodule#Documentation/git-submodule.txt---checkout

dist/index.js

@@ -796,7 +796,7 @@ class GitCommandManager {
     submoduleUpdate(fetchDepth, recursive) {
         return __awaiter(this, void 0, void 0, function* () {
             const args = ['-c', 'protocol.version=2'];
-            args.push('submodule', 'update', '--init', '--force');
+            args.push('submodule', 'update', '--init', '--force', '--checkout');
             if (fetchDepth > 0) {
                 args.push(`--depth=${fetchDepth}`);
             }

src/git-command-manager.ts

@@ -411,7 +411,7 @@ class GitCommandManager {
 
   async submoduleUpdate(fetchDepth: number, recursive: boolean): Promise<void> {
     const args = ['-c', 'protocol.version=2']
-    args.push('submodule', 'update', '--init', '--force')
+    args.push('submodule', 'update', '--init', '--force', '--checkout')
     if (fetchDepth > 0) {
       args.push(`--depth=${fetchDepth}`)
     }