define backport commit message in constant

.github/update-release-branch.py

@@ -13,6 +13,9 @@ No user facing changes.
 
 """
 
+# NB: This exact commit message is used to find commits for reverting during backports.
+# Changing it requires a transition period where both old and new versions are supported.
+BACKPORT_COMMIT_MESSAGE = 'Update version and changelog for v'
 
 # Name of the remote
 ORIGIN = 'origin'
@@ -287,7 +290,7 @@ def main():
     # This commit will not exist the first time we release the v{N-1} branch from the v{N} branch, so we
     # use `git log --grep` to conditionally revert the commit.
     print('Reverting the version number and changelog updates from the last release to avoid conflicts')
-    vOlder_update_commits = run_git('log', '--grep', '^Update version and changelog for v', '--format=%H').split()
+    vOlder_update_commits = run_git('log', '--grep', f'^{BACKPORT_COMMIT_MESSAGE}', '--format=%H').split()
 
     if len(vOlder_update_commits) > 0:
       print(f'  Reverting {vOlder_update_commits[0]}')
@@ -329,7 +332,7 @@ def main():
 
     # Amend the commit generated by `npm version` to update the CHANGELOG
     run_git('add', 'CHANGELOG.md')
-    run_git('commit', '-m', f'Update version and changelog for v{version}')
+    run_git('commit', '-m', f'{BACKPORT_COMMIT_MESSAGE}{version}')
   else:
     # If we're performing a standard release, there won't be any new commits on the target branch,
     # as these will have already been merged back into the source branch. Therefore we can just