Doing git tree lookups based on the SHA-1 of the Message-ID
is expensive as trees get larger, instead, use the SHA-1
object ID directly. This drastically reduces the amount
of time spent in the "git cat-file --batch" process for
fetching the /$INBOX/all.mbox.gz endpoint on the ~800MB
git@vger.kernel.org mirror
This retains backwards compatibility and allows existing
indices to be transparently upgraded without performance
degradation.