builtin/verify-tag.c: ignore SIGPIPE in gpg-interface

builtin/verify-tag.c: ignore SIGPIPE in gpg-interface

The verify_signed_buffer() function may trigger a SIGPIPE when the
GPG child process terminates early (due to a bad keyid, for example)
and Git tries to write to it afterwards.  Previously, ignoring
SIGPIPE was done in builtin/verify-tag.c to avoid this issue.

However, any other caller who wants to call verify_signed_buffer()
would have to do the same.

Use sigchain_push(SIGPIPE, SIG_IGN) in verify_signed_buffer(),
pretty much like in sign_buffer(), so that any caller is not
required to perform this task.

This will avoid possible mistakes by further developers using
verify_signed_buffer().

Signed-off-by: Santiago Torres <santiago@nyu.edu>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>