Fix build with ghc 8.4+, which broke due to the Semigroup Monoid change

https://prime.haskell.org/wiki/Libraries/Proposals/SemigroupMonoid

I am not happy with the fragile pile of CPP boilerplate required to support
ghc back to 7.0, which git-annex still targets for both the android build
and the standalone build targeting old linux kernels. It makes me unlikely
to want to use Semigroup more in git-annex, because the benefit of the
abstraction is swamped by the ugliness. I actually considered ripping out
all the Semigroup instances, but some are needed to use
optparse-applicative.

The problem, I think, is they made this transaction on too fast a timeline.
(Although ironically, work on it started in 2015 or earlier!)
In particular, Debian oldstable is not out of security support, and it's
not possible to follow the simpler workarounds documented on the wiki and
have it build on oldstable (because the semigroups package in it is too
old).

I have only tested this build with ghc 8.2.2, not the newer and older
versions that branches of the CPP support. So there could be typoes, we'll
see.

This commit was sponsored by Brock Spratlen on Patreon.

Types/Difference.hs

@@ -5,6 +5,8 @@
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
+{-# LANGUAGE CPP #-}
+
 module Types.Difference (
 	Difference(..),
 	Differences(..),
@@ -23,8 +25,11 @@ import qualified Git.Config
 
 import Data.Maybe
 import Data.Monoid
-import Prelude
 import qualified Data.Set as S
+#if MIN_VERSION_base(4,9,0)
+import qualified Data.Semigroup as Sem
+#endif
+import Prelude
 
 -- Describes differences from the v5 repository format.
 --
@@ -67,14 +72,27 @@ instance Eq Differences where
 		, oneLevelBranchHash
 		]
 
+appendDifferences :: Differences -> Differences -> Differences
+appendDifferences a@(Differences {}) b@(Differences {}) = a
+	{ objectHashLower = objectHashLower a || objectHashLower b
+	, oneLevelObjectHash = oneLevelObjectHash a || oneLevelObjectHash b
+	, oneLevelBranchHash = oneLevelBranchHash a || oneLevelBranchHash b
+	}
+appendDifferences _ _ = UnknownDifferences
+
+#if MIN_VERSION_base(4,9,0)
+instance Sem.Semigroup Differences where
+	(<>) = appendDifferences
+#endif
+
 instance Monoid Differences where
 	mempty = Differences False False False
-	mappend a@(Differences {}) b@(Differences {}) = a
-		{ objectHashLower = objectHashLower a || objectHashLower b
-		, oneLevelObjectHash = oneLevelObjectHash a || oneLevelObjectHash b
-		, oneLevelBranchHash = oneLevelBranchHash a || oneLevelBranchHash b
-		}
-	mappend _ _ = UnknownDifferences
+#if MIN_VERSION_base(4,11,0)
+#elif MIN_VERSION_base(4,9,0)
+	mappend = (Sem.<>)
+#else
+	mappend = appendDifferences
+#endif
 
 readDifferences :: String -> Differences
 readDifferences = maybe UnknownDifferences mkDifferences . readish