changes: Minor fix to error log in GitPoller._process_changes.

When an error occured while retrieving a rev information (author/timestamp/...)
the error log would log the new HEAD rev of the branch instead of the problematic rev.

master/buildbot/changes/gitpoller.py

@@ -485,7 +485,7 @@ def _process_changes(self, newRev, branch):
             failures = [r[1] for r in results if not r[0]]
             if failures:
                 for failure in failures:
-                    log.err(failure, f"while processing changes for {newRev} {branch}")
+                    log.err(failure, f"while processing changes for {rev} {branch}")
                 # just fail on the first error; they're probably all related!
                 failures[0].raiseException()