Add support for `depth` option in wrap-git

This allows cloning subprojects shallowly.  It works recursively for a
subproject's submodules in case `clone-recursive` is set to `true`.

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

@@ -143,6 +143,14 @@ automatically by adding the following *(since 0.48.0)*:
 clone-recursive = true
 ```
 
+Setting the clone depth is supported using the `depth` directive *(since 0.52.0)*.
+Note that git always allow shallowly cloning branches, but in order to clone commit ids
+shallowly, the server must support `uploadpack.allowReachableSHA1InWant=true`.
+
+```ini
+depth = 1
+```
+
 ## Using wrapped projects
 
 Wraps provide a convenient way of obtaining a project into your subproject directory. 

docs/markdown/snippets/depth.md

@@ -0,0 +1,7 @@
+## Add `depth` option to `wrap-git`
+
+To allow shallow cloning, an option `depth` has been added to `wrap-git`.
+This applies recursively to submodules when `clone-recursive` is set to `true`.
+
+Note that the git server may have to be configured to support shallow cloning
+not only for branches but also for tags.

mesonbuild/wrap/wrap.py

@@ -226,16 +226,21 @@ class Resolver:
             self.apply_patch()
 
     def get_git(self):
+        # git doesn't support directly cloning shallowly for commits,
+        # so we follow https://stackoverflow.com/a/43136160
+        subprocess.check_call(['git', 'init', self.directory], cwd=self.subdir_root)
+        subprocess.check_call(['git', 'remote', 'add', 'origin', self.wrap.get('url')],
+                              cwd=self.dirname)
+        git_options = []
         revno = self.wrap.get('revision')
-        subprocess.check_call(['git', 'clone', self.wrap.get('url'),
-                               self.directory], cwd=self.subdir_root)
-        if revno.lower() != 'head':
-            if subprocess.call(['git', 'checkout', revno], cwd=self.dirname) != 0:
-                subprocess.check_call(['git', 'fetch', self.wrap.get('url'), revno], cwd=self.dirname)
-                subprocess.check_call(['git', 'checkout', revno], cwd=self.dirname)
+        if self.wrap.values.get('depth', '') != '':
+            git_options.extend(['--depth', self.wrap.values.get('depth')])
+        subprocess.check_call(['git', 'fetch'] + git_options + ['origin', revno],
+                              cwd=self.dirname)
+        subprocess.check_call(['git', 'checkout', revno], cwd=self.dirname)
         if self.wrap.values.get('clone-recursive', '').lower() == 'true':
             subprocess.check_call(['git', 'submodule', 'update',
-                                   '--init', '--checkout', '--recursive'],
+                                   '--init', '--checkout', '--recursive'] + git_options,
                                   cwd=self.dirname)
         push_url = self.wrap.values.get('push-url')
         if push_url: