Draft: Optimize gitclone cache/update behavior and enforce pre-conditions on shared.

Mostly to speed up and reduce the access to git server (changes only for gitclone):

• use-cache with explicit revision tries network cache before cloning; on hit only git reset runs
• shared requires revision to be set, otherwise prints warning and disables shared
• git fetch in update() respects --depth option
• upload_network_cached guarded by self.use_cache and self.revision
• update() skips fetch when revision is explicitly provided
• Extra tests added to conver changes and prevent regressions