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>