Improve mount performance by moving validation to mount process and parallelizing I/O

[tyrielv]

Problem

gvfs mount was slower than necessary because gvfs.exe performed substantial validation before launching gvfs.mount.exe, and then the mount process repeated some of it:

• Redundant work: Auth, index parsing, and git repo validation ran in both processes
• Sequential network I/O: Auth/config queries blocked local validations that don't depend on the network
• Redundant HTTP requests: Anonymous probe + separate config query = two round-trips when one suffices

On a large repo (~2.4M index entries), production mount took ~29s.

Three incremental commits

1. Move validation from gvfs.exe → gvfs.mount.exe — strip the verb down to pre-launch checks only (~29s → ~22s)
2. Parallelize local validations with network I/O — run auth+config concurrently with git/hooks/filesystem checks (~22s → ~19s)
3. Combine auth + config into one HTTP flow — save a round-trip by probing with the config query itself (~19s → ~18s, −38% total)

Testing

All unit tests and CI functional tests pass; manual mount/unmount verified on large repo.