Introduce a column marking a repository as empty/new/unsynchronized

in order to help direct history synchronizers to the most appropriate logic, we should introduce a simple boolean field onto {versioncontrol_repositories} that tracks whether or not an initial sync has been performed. i would prefer to do it this way than via heuristics (e.g., checking to see if $repo->loadCommits() and $repo->loadBranches() and $repo->loadTags() are all empty) as it's more efficient, and will make it cleaner to let backends extend with additional data types that could make them non-empty (e.g., notes in git).

once the field is in, we should make all generic logic check this flag when preparing to perform a sync, and redirect to syncInitial() if it is set.

this will mean updating our tests on the Git backend, though - test repositories will probably need to force this to 0 in order to pretend that an initialSync() has occurred so that simulated pushes pass through syncEvent() as they should.