Fix error when script is run after the final tag is created.

(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