Using tag_format without {release} breaks tito tag --undo

[michalfabik]

How to reproduce:

$ echo "tag_format = {version}" >> .tito/tito.props
$ git tag 0.1.6 myproject-0.1.6-1        # provide tito with a latest tag in the new format
$ tito tag --use-version=0.1.7
Creating output directory: /tmp/tito
Tagging new version of myproject: 0.1.6-1 -> 0.1.7-1
Created tag: 0.1.7
   View: git show HEAD
   Undo: tito tag -u
   Push: git push --follow-tags origin
$ tito tag -u
Creating output directory: /tmp/tito
Undoing tag: 0.1.7-1
ERROR: Cannot undo tag that does not exist locally.
$  

Related: #277
Tested with tito 0.6.16