Bug 1558363 - Fix WPT manifest updates when within a git tree r=jgraham

This has been broken since bug 1545629 landed, when we started using
git to (quickly) get hashes of unchanged files. Unfortunately, the
logic for finding changed files only worked when the test root was the
root of the git repository.

Differential Revision: https://phabricator.services.mozilla.com/D34702

--HG--
extra : moz-landing-system : lando

testing/web-platform/tests/tools/manifest/vcs.py

@@ -41,23 +41,12 @@ class GitHasher(object):
 
     def _local_changes(self):
         """get a set of files which have changed between HEAD and working copy"""
-        changes = set()
 
-        cmd = ["status", "-z", "--ignore-submodules=all"]
+        # note that git runs the command with tests_root as the cwd, which may
+        # not be the root of the git repo (e.g., within a browser repo)
+        cmd = [b"diff-index", b"--relative", b"--no-renames", b"--name-only", b"-z", b"HEAD"]
         data = self.git(*cmd)
-
-        in_rename = False
-        for line in data.split(b"\0")[:-1]:
-            if in_rename:
-                changes.add(line)
-                in_rename = False
-            else:
-                status = line[:2]
-                if b"R" in status or b"C" in status:
-                    in_rename = True
-                changes.add(line[3:])
-
-        return changes
+        return set(data.split(b"\0"))
 
     def hash_cache(self):
         # type: () -> Dict[str, Optional[str]]
@@ -66,6 +55,8 @@ class GitHasher(object):
         """
         hash_cache = {}
 
+        # note that git runs the command with tests_root as the cwd, which may
+        # not be the root of the git repo (e.g., within a browser repo)
         cmd = ["ls-tree", "-r", "-z", "HEAD"]
         local_changes = self._local_changes()
         for result in self.git(*cmd).split("\0")[:-1]: