cmd/version, etc: fix version number detection stuff.

cmd/version, etc: fix version number detection stuff.

Gabriel Filion pointed out that bup's version number (which we added to the
man pages automatically) was not detected when you used a bup tarball
generated from 'git archive' (or more likely, you let GitHub call 'git
archive' for you).  That makes sense, since our version detection was based
on having a .git directory around, which the tarball doesn't.

Instead, let's create a .gitattributes file and have it auto-substitute some
version information during 'git archive'.  Of course, if we actually *do*
have a .git directory, continue to use that.

While we're here, add a new 'bup version' command and alias "bup --version"
and "bup -V" to call it, since those options are pretty standard.