scap clean: hide remote line on pushing deletes
When pushing the branch deletions, git relays the status line emitted by Gerrit showing the progress:
remote: remote: Processing changes: refs: 1 remote: Processing changes: refs: 1, done
cgit prints them to stderr and they end up captured in the weekly email we receive (T303828#10457972).
Capture stderr and only print it when a push fails.
Bug: T303828