Merge branch 'jk/epipe-in-async'

Merge branch 'jk/epipe-in-async'

Handling of errors while writing into our internal asynchronous
process has been made more robust, which reduces flakiness in our
tests.

* jk/epipe-in-async:
  t5504: handle expected output from SIGPIPE death
  test_must_fail: report number of unexpected signal
  fetch-pack: ignore SIGPIPE in sideband demuxer
  write_or_die: handle EPIPE in async threads