git: fix leaking system paths

git: fix leaking system paths

Git has some flags to make it output system paths as they have been
compiled into Git. This is done by calling `system_path()`, which
returns an allocated string. This string isn't ever free'd though,
creating a memory leak.

Plug those leaks. While they are surfaced by t0211, there are more
memory leaks looming exposed by that test suite and it thus does not yet
pass with the memory leak checker enabled.

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