sideband: fix leaks when configuring sideband colors

sideband: fix leaks when configuring sideband colors

We read a bunch of configs in `use_sideband_colors()` to configure the
colors that Git should use. We never free the strings read from the
config though, causing memory leaks.

Refactor the code to use `git_config_get_string_tmp()` instead, which
does not allocate memory. As we throw the strings away after parsing
them anyway there is no need to use allocated strings.

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