Add a to-do item suggested by Tony Finch.
I hate my typos.
Instrument to find out where unnumbered heads are coming from.
Remove compute_parent_links and fileop_sort

Generate fileops directly from a merge join between files
in child commit and parent commit.

This has two benefits to computing parent links first.

1. We only iterate each file list once
2. The generated fileops are in the same order as the input
   files, which is also the order we want to output them
   so no additional sort step is required.

Note, the files are now sorted by output name instead of input name.
This affects the position of .cvsignore files, as they are renamed
to .gitignore.

This takes eamcs fast output from 15s to 8.5s

Over the course of the first merge join patch and this,
netbsd-pkgsrc fast output went from 1179s to 835s to 477s