test: add test-transaction-rollback-on-sigpipe.t demonstrating py3 regression When an hg push is interrupted with C-c, the remote [hg serve] command receives SIGPIPE. If a pretxnchangegroup hook fails, the remote hg then tries to rollback the transaction. It begins by printing "transaction abort!\n". This returns EPIPE, but ui.py ignores that error. In python3 (but not python2), this "transaction abort!\n" message stays in a buffer, so future flushes of stderr will try to print the message again, and so those flushes will also hit EPIPE. This test demonstrates such a case where this EPIPE causes the transaction rollback to fail, leaving behind an abandoned transaction. Differential Revision: https://phab.mercurial-scm.org/D9151