split out setEnv to avoid adding dep

Windows needs the setenv package in custom-setup, but I don't want to
pull it in on unix, which would probably break some builds and need more
work. Instead, split out setEnv to a separate module.

Quite likely, unix-compat will get a portable environment layer, and
then both modules can be removed from here.

This commit was sponsored by Øyvind Andersen Holm.

Git/CurrentRepo.hs

@@ -12,6 +12,7 @@ import Git.Types
 import Git.Construct
 import qualified Git.Config
 import Utility.Env
+import Utility.Env.Set
 
 {- Gets the current git repository.
  -