[update-checkout] Gracefully degrade time match.

Previously, when invoking the script with match-timestamp, if the
refspec was absent, the find_rev_by_timestamp function would throw an
exception and the script would hang forever.

Here, it is first checked via `git rev-parse --verify $REFSPEC` that the
refspec actually exists in the target repo.  If it does not, the refspec
is just omitted from the command, giving the latest commit before the
currently checked out one.

Author: nate-chandler

utils/update_checkout/update_checkout/update_checkout.py

@@ -100,8 +100,15 @@ def confirm_tag_in_repo(tag, repo_name) -> Optional[str]:
 
 
 def find_rev_by_timestamp(timestamp, repo_name, refspec):
+    refspec_exists = True
+    try:
+        shell.run(["git", "rev-parse", "--verify", refspec])
+    except Exception:
+        refspec_exists = False
     args = ["git", "log", "-1", "--format=%H", "--first-parent",
-            '--before=' + timestamp, refspec]
+            '--before=' + timestamp]
+    if refspec_exists:
+        args.append(refspec)
     rev = shell.capture(args).strip()
     if rev:
         return rev