v2: index forwards (via `git log --reverse')

v2: index forwards (via `git log --reverse')

Since we'll need to expose THREADID to JMAP and IMAP users,
index all messages in the order they were committed to ensure
our `tid' (thread ID) column ascends in mirrors the same way
they do in the source inbox.

This drastically simplifies our code but increases memory
usage of `git-log'.  The next commit will bring memory use
back down at the expense of $TMPDIR usage.