repair: clarify in comment that caller must take lock, but not transaction

I have checked that all callers have already taken the lock (and if
they hadn't, we should have seen tests fail thanks to the 'transaction
requires locking' devel warning in localrepo.transaction()).

mercurial/repair.py

@@ -76,8 +76,9 b' def _collectbrokencsets(repo, files, str'
     return s
 
 def strip(ui, repo, nodelist, backup=True, topic='backup'):
-    # This function operates within a transaction of its own, but does
-    # not take any lock on the repo.
+    # This function requires the caller to lock the repo, but it operates
+    # within a transaction of its own, and thus requires there to be no current
+    # transaction when it is called.
     # Simple way to maintain backwards compatibility for this
     # argument.
     if backup in ['none', 'strip']: