broke out Git/Branch.hs and reorganized

Git/Branch.hs

@@ -0,0 +1,60 @@
+{- git branch stuff
+ -
+ - Copyright 2011 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Git.Branch where
+
+import qualified Data.ByteString.Lazy.Char8 as L
+
+import Common
+import Git
+
+{- Checks if the second branch has any commits not present on the first
+ - branch. -}
+changed :: Branch -> Branch -> Repo -> IO Bool
+changed origbranch newbranch repo
+	| origbranch == newbranch = return False
+	| otherwise = not . L.null <$> diffs
+	where
+		diffs = Git.pipeRead
+			[ Param "log"
+			, Param (show origbranch ++ ".." ++ show newbranch)
+			, Params "--oneline -n1"
+			] repo
+
+{- Given a set of refs that are all known to have commits not
+ - on the branch, tries to update the branch by a fast-forward.
+ -
+ - In order for that to be possible, one of the refs must contain
+ - every commit present in all the other refs.
+ -}
+fastForward :: Branch -> [Ref] -> Repo -> IO Bool
+fastForward _ [] _ = return True
+fastForward branch (first:rest) repo = do
+	-- First, check that the branch does not contain any
+	-- new commits that are not in the first ref. If it does,
+	-- cannot fast-forward.
+	diverged <- changed first branch repo
+	if diverged
+		then no_ff
+		else maybe no_ff do_ff =<< findbest first rest
+	where
+		no_ff = return False
+		do_ff to = do
+			Git.run "update-ref"
+				[Param $ show branch, Param $ show to] repo
+			return True
+		findbest c [] = return $ Just c
+		findbest c (r:rs)
+			| c == r = findbest c rs
+			| otherwise = do
+			better <- changed c r repo
+			worse <- changed r c repo
+			case (better, worse) of
+				(True, True) -> return Nothing -- divergent fail
+				(True, False) -> findbest r rs -- better
+				(False, True) -> findbest c rs -- worse
+				(False, False) -> findbest c rs -- same