Fix error when script is run after the final tag is created. (#18638)

(This script is run at different points in the release process, including once after the tag is created to update the final release index page and GitHub release page.)

PiperOrigin-RevId: 537860711
Change-Id: I6bf129c82eb2c83b37993d4ef691d4783c5735b6

scripts/release/relnotes.py

@@ -171,13 +171,13 @@ def get_external_authors_between(base, head):
  is_major = bool(re.fullmatch(r"\d+.0.0", current_release))
 
  tags = [tag for tag in git("tag", "--sort=refname") if "pre" not in tag]
+
+ # Get the baseline for RCs (before release tag is created)
  if current_release not in tags:
  tags.append(current_release)
- tags.sort()
- last_release = tags[tags.index(current_release) - 1]
- else:
- print("Error: release tag already exists")
- sys.exit(1)
+
+ tags.sort()
+ last_release = tags[tags.index(current_release) - 1]
 
  # Assuming HEAD is on the current (to-be-released) release, find the merge
  # base with the last release so that we know which commits to generate notes