* rev-parse --short without any argument defaults to 8 chars
* truncation is absolutely moot here
* rev-list --abbrev-commit also uses 8 chars by default
because of length mismatch (COMMIT - 9 chars, TAGGEDCOMMIT - 8 chars) it always was mismatching them even when it should've been detecting that it's at release tag.
cut out explicit lengths and leave everything to use defaults, because they work (they both rely on the same core.abbrev setting so no length mismatch should happen).
This rewrites the version info using lokid's approach of compiling it
into a .cpp file that gets generated as part of the build (*not* during
the configure stage).
Among other things, this means that changing the version no longer
invalidates ccache or cmake dependencies, and because it depends on
`.git/index` git commits will cause the version to be regenerated,
making the commit tag more reliable (currently if you rebuild without
running cmake your git commit tag doesn't update).
