Tweak base/version.jl printing a bit

* I don't like printing "Unknown commit" when there's not `git` information, just print out what we know and leave it at that

* versioninfo() depended on the `commit_string` variable, but that gets changed when on a tagged release.  Now it just builds its own `commit_string` internally.  One less weird global variable floating around!
Merge pull request #4743 from JuliaLang/sf/versiontweaks

Tweak base/version.jl printing a bit