Raise exception if invalid values are given to 'clone-recursive'

Fixes #13821 but perhaps a separate ticket should be opened for a
related bug, or maybe some extra docs are needed. Note in the ticket I
mentioned three things I had to do to get it working as expected after
the failures.

docs/markdown/Release-notes-for-0.48.0.md

@@ -310,7 +310,7 @@ files get exported.
 To enable this, the following needs to be added to the `.wrap` file:
 
 ```ini
-clone-recursive=true
+clone-recursive = true
 ```
 
 ## `subproject()` function now supports the `required:` kwarg

docs/markdown/Wrap-dependency-system-manual.md

@@ -135,7 +135,7 @@ case, the directory will be copied into `subprojects/` before applying patches.
 - `push-url` - alternative url to configure as a git push-url. Useful if
   the subproject will be developed and changes pushed upstream.
   *(since 0.37.0)*
-- `clone-recursive` - also clone submodules of the repository
+- `clone-recursive = true` - also clone submodules of the repository
   *(since 0.48.0)*
 
 ## wrap-file with Meson build patch

mesonbuild/wrap/wrap.py

@@ -600,6 +600,9 @@ def _get_git(self, packagename: str) -> None:
         if self.wrap.values.get('depth', '') != '':
             is_shallow = True
             depth_option = ['--depth', self.wrap.values.get('depth')]
+        clone_recursive_val = self.wrap.values.get('clone-recursive', '').lower()
+        if clone_recursive_val and clone_recursive_val not in ['false', 'true']:
+            raise WrapException(f"clone-recursive: invalid value '{clone_recursive_val}' (use 'true' or 'false')")
         # for some reason git only allows commit ids to be shallowly fetched by fetch not with clone
         if is_shallow and self.is_git_full_commit_id(revno):
             # git doesn't support directly cloning shallowly for commits,
@@ -622,7 +625,7 @@ def _get_git(self, packagename: str) -> None:
                     args += ['--branch', revno]
                 args += [self.wrap.get('url'), self.directory]
                 verbose_git(args, self.subdir_root, check=True)
-        if self.wrap.values.get('clone-recursive', '').lower() == 'true':
+        if clone_recursive_val == 'true':
             verbose_git(['submodule', 'update', '--init', '--checkout', '--recursive', *depth_option],
                         self.dirname, check=True)
         push_url = self.wrap.values.get('push-url')