t5528: do not fail with FreeBSD shell

t5528: do not fail with FreeBSD shell

The FreeBSD shell converts this expression:

  git ${1:+-c push.default="$1"} push

to this when "$1" is not empty:

  git "-c push.default=$1" push

which causes git to fail.  To avoid this we simply break up the
expansion into two parts so that the whitespace which creates
two arguments instead of one is outside the ${...} like so:

  git ${1:+-c} ${1:+push.default="$1"} push

This has the desired effect on all platforms allowing the test
to pass on FreeBSD.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>