Fix incorrect version match bug

Adding the extra characters are causing tb to match tagged versions that
have been modified.  For example, Mithril `v0.1.2` was also matching
`v0.1.2-2-g2a0e6ea` after adding arbitrary extra characters.  This was
causing not only an incorrect match but also for `v0.1.2` to be
inaccessible.  This change requires that the user specify the exact rev
they want, which is actually a good thing.  If their rev doesn't exist,
they should get an error not an incorrect version.