Repository tuning parameters can now be passed when initializing a repository for the first time.

* init: Repository tuning parameters can now be passed when initializing a
  repository for the first time. For details, see
  http://git-annex.branchable.com/tuning/
* merge: Refuse to merge changes from a git-annex branch of a repo
  that has been tuned in incompatable ways.

Annex/Init.hs

@@ -27,6 +27,7 @@ import Logs.UUID
 import Logs.Trust.Basic
 import Types.TrustLevel
 import Annex.Version
+import Annex.Difference
 import Annex.UUID
 import Config
 import Annex.Direct
@@ -73,6 +74,7 @@ initialize' = do
 	checkCrippledFileSystem
 	unlessM isBare $
 		hookWrite preCommitHook
+	setDifferences
 	setVersion supportedVersion
 	ifM (crippledFileSystem <&&> not <$> isBare)
 		( do