multi_git: hoist out common epoch/alternates handling
IMHO, this greatly improves code sharing and organization
between v2, extindex, and lei/store. Common git-related
logic for these is lightly-refactored and easier to reason
about.
The impetus for this big change was to ensure inboxes
created+managed by public-inbox-{clone,fetch} could have
alternates and configs setup properly without depending on
SQLite (via V2Writable). This change does that while
making old code shorter and better factored.