project: accept v3 git bundles in _IsValidBundle

Git 2.29+ introduced v3 bundle headers (`# v3 git bundle`) for SHA-256
and filtered bundles. Allow both v2 and v3 bundle headers when
validating downloaded `clone.bundle` files.

Bug: 576377949
Change-Id: Ic5ac94bf535a801d948f8932213e62a6cd132cac
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/647123
Tested-by: Brian Gan <brgan@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Brian Gan <brgan@google.com>

project.py

@@ -3874,7 +3874,7 @@ class Project:
     def _IsValidBundle(self, path, quiet):
         try:
             with open(path, "rb") as f:
-                if f.read(16) == b"# v2 git bundle\n":
+                if f.read(16) in (b"# v2 git bundle\n", b"# v3 git bundle\n"):
                     return True
                 else:
                     if not quiet: