git_config: recognize only complete object IDs

Full Git object IDs are exactly 40 hexadecimal digits for SHA-1 or 64
for SHA-256. Stop treating every intermediate length as immutable, which
could bypass normal ref resolution for invalid revision strings.

Bug: 553599402
Change-Id: I41b4ce6b2bfe2a2d8b351c7f040ec0cf469b474b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/623901
Reviewed-by: Brian Gan <brgan@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

git_config.py

@@ -40,7 +40,7 @@ from repo_trace import Trace
 # that is saved in the config.
 SYNC_STATE_PREFIX = "repo.syncstate."
 
-ID_RE = re.compile(r"^[0-9a-f]{40,64}$")
+ID_RE = re.compile(r"^(?:[0-9a-f]{40}|[0-9a-f]{64})$")
 
 REVIEW_CACHE = {}