Add support for .hg_archival to detect_vcs

This allows vcs_tag() to work on a mercurial archived tarball so programs can
display the revision at runtime.

mesonbuild/utils/universal.py

@@ -772,6 +772,14 @@ def detect_vcs(source_dir: T.Union[str, Path]) -> T.Optional[T.Dict[str, str]]:
             'rev_regex': '(.*)',
             'dep': '.hg/dirstate'
         },
+        {
+            'name': 'mercurial',
+            'cmd': 'cat',
+            'repo_dir': '.',
+            'get_rev': 'cat .hg_archival.txt',
+            'rev_regex': 'node: (.*)\n',
+            'dep': '.hg_archival.txt'
+        },
         {
             'name': 'subversion',
             'cmd': 'svn',