Ignore SIGPIPE when running a filter driver

Ignore SIGPIPE when running a filter driver

If a filter is not defined or if it fails, git should behave as if the
filter is a no-op passthru.

However, if the filter exits before reading all the content, depending on
the timing, git could be killed with SIGPIPE when it tries to write to the
pipe connected to the filter.

Ignore SIGPIPE while processing the filter to give us a chance to check
the return value from a failed write, in order to detect and act on this
mode of failure in a more controlled way.

Signed-off-by: Jehan Bing <jehan@orb.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>