provide a default String to avoid propagating TH depends

[juhp]

I think most consumers of this library want it for including git info in their --version output.
So if gitrev provided some generic git hash String, they could use that to avoid having to depend on and use template-haskell directly.
Would probably have to look through how packages are currently using gitrev to come up with a sane useful default or two.

[juhp]

Nevermind I am not thinking clearly: that is not possible of course.