Do not over-quote the -f envelopesender value.

Do not over-quote the -f envelopesender value.

Without this, the value passed to sendmail would have an extra set of
single quotes.  At least exim's sendmail emulation would object to that:

    exim: bad -f address "'list-addr@example.org'": malformed address: ' \
      may not follow 'list-addr@example.org
    error: hooks/post-receive exited with error code 1

Signed-off-by: Jim Meyering <jim@meyering.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>