[tools] check format of target branch in roll_merge_gerrit.py

This prevents accidental use for release branch merging, which
works but shouldn't.

Change-Id: I4db99bb721c935a8a1c7c44c1b4d909f44a8bf9c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3705382
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81157}

tools/release/roll_merge_gerrit.py

@@ -63,6 +63,9 @@ def main():
 
   # Get the original commit.
   revision = options.revision[0]
+  if not re.match(r"[0-9]+\.[0-9]+\.[0-9]+", options.branch):
+    print("Branch is not of the form 1.2.3")
+    exit(1)
   print("Cherry-picking %s onto %s" % (revision, options.branch))
 
   # Create a cherry pick commit from the original commit.