Merge branch 'jc/ignore-epipe-in-filter' into next

Merge branch 'jc/ignore-epipe-in-filter' into next

Filter scripts were run with SIGPIPE disabled on the Git side,
expecting that they may not read what Git feeds them to filter.
We however treated a filter that does not read its input fully
before exiting as an error.

This changes semantics, but arguably in a good way.  If a filter
can produce its output without consuming its input using whatever
magic, we now let it do so, instead of diagnosing it as a
programming error.

* jc/ignore-epipe-in-filter:
  filter_buffer_or_fd(): ignore EPIPE
  copy.c: make copy_fd() report its status silently