enable LambdaCase and convert around 10% of places that could use it

Needs ghc 7.6.1, so minimum base version increased slightly. All builds
are well above this version of ghc, and debian oldstable is as well.

Code that could use lambdacase can be found by running:
git grep -B 1 'case ' | less
and searching in less for "<-"

This commit was sponsored by andrea rota.

Annex/MetaData.hs

@@ -39,8 +39,7 @@ import Data.Time.Clock.POSIX
  -}
 genMetaData :: Key -> FilePath -> FileStatus -> Annex ()
 genMetaData key file status = do
-	v <- catKeyFileHEAD file
-	case v of
+	catKeyFileHEAD file >>= \case
 		Nothing -> noop
 		Just oldkey -> 
 			whenM (copyMetaData oldkey key)