migrate --update gets keys

migrate --update gets keys

The git log is outputting the diff, but this only looks at the new
files. When we have a new file, we can get the old filename by just
replacing "new" with "old". And then use branchFileRef to refer to it
allows catting the old key.

While this does have to skip past the old files in the diff, it's still
faster than calling git diff separately.

Sponsored-by: Nicholas Golder-Manning on Patreon