newvers.sh: Speed up git_tree_modified

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