msubprojects: Support git subprojects with no wrap file

User could have cloned manually a subproject.

mesonbuild/msubprojects.py

@@ -53,6 +53,10 @@ def update_git(wrap, repo_dir, options):
         mlog.log('  -> Not used.')
         return
     revision = wrap.get('revision')
+    if not revision:
+        # It could be a detached git submodule for example.
+        mlog.log('  -> No revision specified.')
+        return
     branch = git_output(['branch', '--show-current'], repo_dir).strip()
     if branch == '':
         try:
@@ -145,6 +149,9 @@ def checkout(wrap, repo_dir, options):
     if wrap.type != 'git' or not os.path.isdir(repo_dir):
         return
     branch_name = options.branch_name if options.branch_name else wrap.get('revision')
+    if not branch_name:
+        # It could be a detached git submodule for example.
+        return
     cmd = ['checkout', branch_name, '--']
     if options.b:
         cmd.insert(1, '-b')

mesonbuild/wrap/wrap.py

@@ -118,10 +118,22 @@ class PackageDefinition:
         self.provided_deps[self.name] = None
         if fname.endswith('.wrap'):
             self.parse_wrap(fname)
+        else:
+            self.guess_type();
         self.directory = self.values.get('directory', self.name)
         if os.path.dirname(self.directory):
             raise WrapException('Directory key must be a name and not a path')
 
+    def guess_type(self) -> None:
+        if os.path.exists(os.path.join(self.filename, '.git')):
+            # This is a git subproject without wrap file. Either the user cloned
+            # it manually, or it's a git submodule. The revision is used in
+            # msubprojects.py to update the git repo. If it's a submodule the repo
+            # is likely detached and revision will be empty.
+            res, stdout = quiet_git(['branch', '--show-current'], self.filename)
+            self.values['revision'] = stdout.strip()
+            self.type = 'git'
+
     def parse_wrap(self, fname: str) -> None:
         try:
             self.config = configparser.ConfigParser(interpolation=None)