Let gvb binary specify "only look in the local git tree"

Turns out that when you get too clever, there's a strong possibility that
you'll blow your foot off.  While detecting where you're being run from is
cute and all, the way I was doing it was causing *everyone* to get GVB's own
version numbers when running git-version-bump <major|minor|patch>.  Now the
gvb binary can say "only look at the git tree we're in", and everyone is
much happier.