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/Direct.hs

@@ -111,9 +111,8 @@ preCommitDirect = do
 		withkey (DiffTree.srcsha diff) (DiffTree.srcmode diff) removeAssociatedFile
 		withkey (DiffTree.dstsha diff) (DiffTree.dstmode diff) addAssociatedFile
 	  where
-		withkey sha _mode a = when (sha /= nullSha) $ do
-			k <- catKey sha
-			case k of
+		withkey sha _mode a = when (sha /= nullSha) $
+			catKey sha >>= \case
 				Nothing -> noop
 				Just key -> void $ a key $
 					makeabs $ DiffTree.file diff
@@ -427,14 +426,12 @@ setDirect wantdirect = do
 			then moveconfig coreworktree indirectworktree
 			else moveconfig indirectworktree coreworktree
 		setConfig (ConfigKey Git.Config.coreBare) val
-	moveconfig src dest = do
-		v <- getConfigMaybe src
-		case v of
-			Nothing -> noop
-			Just wt -> do
-				unsetConfig src
-				setConfig dest wt
-				reloadConfig
+	moveconfig src dest = getConfigMaybe src >>= \case
+		Nothing -> noop
+		Just wt -> do
+			unsetConfig src
+			setConfig dest wt
+			reloadConfig
 
 {- Since direct mode sets core.bare=true, incoming pushes could change
  - the currently checked out branch. To avoid this problem, HEAD
@@ -474,8 +471,7 @@ switchHEADBack = maybe noop switch =<< inRepo Git.Branch.currentUnsafe
   where
 	switch currhead = do
 		let orighead = fromDirectBranch currhead
-		v <- inRepo $ Git.Ref.sha currhead
-		case v of
+		inRepo (Git.Ref.sha currhead) >>= \case
 			Just headsha
 				| orighead /= currhead -> do
 					inRepo $ Git.Branch.update "leaving direct mode" orighead headsha