Having tens of thousands of inboxes and associated git processes
won't work well, so we'll use libgit2 to access the object DB
directly. We only care about OID lookups and won't need to rely
on per-repo revision names or paths.
The Git::Raw XS package won't be used since its manpages don't
promise a stable API. Since we already use Inline::C and have
experience with I::C when it comes to compatibility, this only
introduces libgit2 itself as a source of new incompatibilities.
This also provides an excuse for me to writev(2) to reduce
syscalls, but liburing is on the horizon for next year.