lei_to_mail: improve SIGPIPE handling

I can't reproduce this in t/lei-sigpipe.t with GIANT_INBOX_DIR.
In real-world usage, having a large `lei q -f text ...' output
piped to a pager and killing the pager prematurely could
trigger:

	non-fatal error from PublicInbox::LeiToMail $?=256

messages in my terminal.  This is because $self->{lei} was
becoming undefined in the process cleanup process of
git_to_mail.  So flip the cleanup logic around and
unconditionally check for Git::cleanup state to bail out
early.

With this change, the `non-fatal error ...' message no longer
appears when I stop reading results early.

0 files changed, 0 insertions, 0 deletions