newvers.sh: Speed up git_tree_modified

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 <nwf20@cl.cam.ac.uk>
Reviewed by: bdrewery, emaste, imp
Obtained from: CheriBSD
MFC after: 3 days
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D27646