newvers.sh: accommodate git worktree

newvers.sh looks for a .vcs subdirectory (e.g. .git, .svn) to determine
which vcs info tool to run (e.g., git rev-parse, svn info).

(As of r308789 if a .vcs subdirectory is not found at ${TOPDIR} then
newvers.sh walks up successive parent directories, testing for the .vcs
subdirectory at each step. This is done in case the FreeBSD source is
built in a subdirectory as part of some larger project, but either way
newvers.sh still tests for the .vcs subdirectory.)

However, when using git worktree there is no .git subdirectory but
rather a plain text .git file which contains a reference to the main
working tree.

Change findvcs() to test that the .vcs entry exists, regardless of type.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation