Offline mode for imported git repositories

[openvmp]

Update the "git" project factory class so that the behavior changes if there is already a cloned copy of the repo:

• if the exact revision is specified and it is a tag, then do nothing
• if the exact revision is specified and it is a branch, then attempt to update it if the last update happened more than T1 time ago
• do not log any error (warning only) if the update attempt failed less than T2 time since the last successful update
• do not log any error and, preferrably, do not waste any time waiting, if it is determined with a high degree of likelihood that there is no Internet connectivity

Maintain default values for T1 (1 hour) and T2 (1 day) and allow the user to override the defaults in the "user config" (user_config.py).
Implement all of that without introducing any non-pure-Python dependencies.

[openvmp]

The only part left is detecting network connectivity. Perhaps a context flag needs to be added to forbid any networking requests in addition to the initial requirements.