Extend the version-distance with an abbreviated form of last commit's SHA.
For example, from
2.1.0.dev4+4c1a7c5 because tags
2.1.0-rcX all yield the same root
2.1.0 so four commits ahead of tag
2.1.0-rc1 and four commits ahead of
2.1.0-rc2 will yield the same version distance
2.1.0.dev4. We need also to append the an abbreviated form of last commit's SHA.