Since all.git (v2) and ALL.git (extindex) encompass every single
epoch or indexed inbox; and is_ancestor() only uses hexadecimal
OIDs; there is no good reason to use $unit->{git} for an
epoch-local $git->check.
This prevents dozens/hundreds of --batch-check processes from
being left running after indexing and can improve locality
if size checks are being done (since that uses --batch-check,
too).
Theoretically several epochs may have conflicting OIDs, but
we're screwed in those cases, anyways, so we might as well
detect it earlier (though I'm not sure what the behavior would
be :x).