send-pack: add gvfs.negativeRefCheck to skip missing negatives

[derrickstolee]

When pushing from a repository that uses the GVFS Protocol, git push spawns git pack-objects --all-progress-implied --revs --stdout --thin -q and feeds it revision parameters on stdin: the advertised refs and negotiated objects as negative (exclusion) tips, plus each ref's old and new tips. Normally feed_object() omits a negative object that is not present locally, but the core.gvfs GVFS_MISSING_OK bit disables that check so that missing negatives are still fed.

That bypass is actively harmful under the GVFS Protocol. pack-objects adds --objects-edge for --thin and, while marking edges uninteresting and hunting for preferred delta bases, reads the tree of every fed exclusion. For an exclusion the client does not have locally, that read lazily downloads the object, issuing one gh_client__get_ immediate request per advertised ref. A scalar clone against a server that advertises many refs therefore triggers a storm of immediate object fetches during an ordinary push.

Partial clone deliberately assumes the objects behind our refs are already present and never fetches them for a push. Restore that behavior for the GVFS Protocol behind a new opt-in config, gvfs.negativeRefCheck. When set, git push performs a non-fetching existence check -- odb_has_object() with flags 0 implies OBJECT_INFO_QUICK | OBJECT_INFO_SKIP_FETCH_OBJECT, so the probe never fetches -- and omits any advertised object it does not have, exactly as Git does without the GVFS_MISSING_OK bit. The decision is computed once in pack_objects() because advertised ref lists can be large.

The config defaults to false, so the legacy GVFS_MISSING_OK behavior is unchanged unless an operator opts in, keeping the fallout controllable.

• This change only applies to interactions with Azure DevOps and the
  GVFS Protocol.