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

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

lockfile.c

@@ -289,7 +289,7 @@ int hold_lock_file_for_append(struct lock_file *lk, const char *path, int flags)
 		int save_errno = errno;
 
 		if (flags & LOCK_DIE_ON_ERROR)
-			exit(128);
+			die("failed to prepare '%s' for appending", path);
 		close(orig_fd);
 		rollback_lock_file(lk);
 		errno = save_errno;