Automatically build merge.lift and fix up the corner cases with sed

reposurgeon doesn't seem to take timestamp indices on the first element
of a set so we have to manually resolve those to the right mark.

Also if a merged branch is deleted, all commits back to the last tag are
lost, so only delete branches with git after we're done with rewriting.
Merged worked out pretty nicely.

Current repacked bare git repository is just under 12MB.

There are additional commits that could probably get collapsed, but the
next big thing is probably going to be validating that the converted
repository actually contains the files we expect it to have at various
points in the history.