newvers: avoid clearing svn revision information with nested VCS dirs

Consider the case where FreeBSD is checked out via Subversion with a
(perhaps unrelated) .git or .hg directory at a higher level - for
example,

.../.git
.../src/freebsd

Previously newvers obtained the SVN revision information via svnversion,
and then tried to obtain the SVN revision corresponding to the git or hg
commit, overwriting the existing information.

As a short term fix use a different variable for hg-svn or git-svn
information, setting $svn from hg or git info only if not empty.

Reported by: Matthias Apitz
Sponsored by: The FreeBSD Foundation