vfs: add the vfs/refresh rc command

vfs/refresh will walk the directory tree for the given paths and
freshen the directory cache. It will use the fast-list capability
of the remote when enabled.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f