Git cache: serve cached repositories offline #728

Follow-up to #403 (step 5 of the plan there).

A cached Git repository is served without the upstream only while its last successful check is younger than git.stale_max_age (24h by default). After that a clone fails, although the repository is on disk. That is right for a proxy in front of a reachable forge, but not for a site that is cut off on purpose, or for an outage that lasts longer than stale_max_age.

An offline mode would serve what the cache holds without contacting the upstream:

• per upstream or for the whole git cache, e.g. git.offline: true
• no ref check and no fetch; branches and tags as last fetched
• a commit the cache doesn't have fails at once instead of waiting on the upstream
• the UI and the log say a repository was served offline, and how old its copy is

Open questions: whether this is a config switch only or can also be turned on at runtime, and whether it should also cover the background refresh from #706 (which would simply not run).