fetch: load all default config at startup

When we start the git-fetch program, we call git_config to load all config, but our callback only processes the fetch.prune option; we do not chain to git_default_config at all. This means that we may not load some core configuration which will have an effect. For instance, we do not load core.logAllRefUpdates, which impacts whether or not we create reflogs in a bare repository. Note that I said "may" above. It gets even more exciting. If we have to transfer actual objects as part of the fetch, then we call fetch_pack as part of the same process. That function loads its own config, which does chain to git_default_config, impacting global variables which are used by the rest of fetch. But if the fetch is a pure ref update (e.g., a new ref which is a copy of an old one), we skip fetch_pack entirely. So we get inconsistent results depending on whether or not we have actual objects to transfer or not! Let's just load the core config at the start of fetch, so we know we have it (we may also load it again as part of fetch_pack, but that's OK; it's designed to be idempotent). Our tests check both cases (with and without a pack). We also check similar behavior for push for good measure, but it already works as expected. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>