newvers.sh: avoid possibly invalid relative directory

Previously newvers.sh passed --work-tree=${VCSDIR}/.. when invoking git.
When using git worktree .git is actually a file, not a directory, and
.git/.. is not a valid path. Although it appears git handles this
internally (perhaps it normalizes the path first), it is simple enough
for the script to store both the working tree top-level directory and
the VCS (.git) directory, so do so.

Sponsored by: The FreeBSD Foundation