builtin/config: convert `key` to a local variable

builtin/config: convert `key` to a local variable

The `key` variable is used by the `get_value()` function for two
purposes:

  - It is used to store the result of `git_config_parse_key()`, which is
    then passed on to `collect_config()`.

  - It is used as a store to convert the provided key to an
    all-lowercase key when `use_key_regexp` is set.

Neither of these cases warrant a global variable at all. In the former
case we can pass the key via `struct collect_config_data`. And in the
latter case we really only want to have it as a temporary local variable
such that we can free associated memory.

Refactor the code accordingly to reduce our reliance on global state.

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