remote: fix leaking config strings

remote: fix leaking config strings

We're leaking several config strings when assembling remotes, either
because we do not free preceding values in case a config was set
multiple times, or because we do not free them when releasing the remote
state. This includes config strings for "branch" sections, "insteadOf",
"pushInsteadOf", and "pushDefault".

Plug those leaks.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>