fetch-pack: split out fsck config parsing

fetch-pack: split out fsck config parsing

When `fetch_pack_config()` is invoked, fetch-pack configuration is
parsed from the config. As part of this operation, fsck message severity
configuration is assigned to the `fsck_msg_types` global variable. This
is optionally used to configure the downstream git-index-pack(1) when
the `--strict` option is specified.

The same parsed fsck message severity configuration is also needed
outside of fetch-pack. Instead of exposing/relying on the existing
global state, split out the fsck config parsing logic into
`fetch_pack_fsck_config()` and expose it. In a subsequent commit, this
is used to provide fsck configuration when invoking `unbundle()`.

For `fetch_pack_fsck_config()` to discern between errors and unhandled
config variables, the return code when `git_config_path()` errors is
changed to a different value also indicating success. This frees up the
previous return code to now indicate the provided config variable
was unhandled. The behavior remains functionally the same.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>