Provides version numbers from calling git on build time or from an embedded string.

The former is based on the gitrev package with a Maybe interface around it, while the embedding is done using a special c-array placeholder in cbits/revision.c

NOTE: gitrev does not register the git state as a compile-time dependency, so the two values below go stale until hydra-prelude happens to be recompiled. That is the trade we want. hydra-prelude is the root of the dependency tree, and a package that does register it (githash registers .git/HEAD, .git/index and .git/packed-refs) rebuilds every package downstream of it after a plain git add. Only dev builds are affected: released binaries carry their revision in the 'cbits/revision.c' placeholder, patched by nix (see nixhydrapackages.nix), and embeddedRevision is preferred over gitRevision at every call site.