newvers.sh: Speed up git_tree_modified

[brooks]

Summary

We're looking for file content differences, so ask the question of git
more directly. This helps a lot, saving tens of thousands of fork()s,
when the builder and editor see different stat() results (e.g., UIDs),
as they might with containers.

Submitted by: Nathaniel Wesley Filardo <[e-mail]>

[emaste]

The old diff-index comes from rS197334 and had many tweaks to address bugs.

I think this change looks good.

[brooks]

CI run https://cirrus-ci.com/task/5615131910471680