newvers.sh: fix git false positive -dirty tag

Assuming that any output from git diff-index --name-only implies
changes in the working tree results in false positives: files with
metadata, but not content, changes are also listed.

Check that content differences exist before adding the -dirty tag to
the git hash.

PR: 229230
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D15968