http: refactor code to clarify memory ownership

http: refactor code to clarify memory ownership

There are various variables assigned via `git_config_string()` and
`git_config_pathname()` which are never free'd. This bug is relatable
because the out parameter of those functions are a `const char **`, even
though memory ownership is transferred to the caller.

We're about to adapt the functions to instead use `char **`. Prepare the
code accordingly. Note that the `(const char **)` casts will go away
once we have adapted the functions.

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