new method for merging changes into adjusted branch that avoids unncessary merge conflicts

Still needs work when there are actual merge conflicts.

Annex/GitOverlay.hs

@@ -0,0 +1,63 @@
+{- Temporarily changing the files git uses.
+ -
+ - Copyright 2014-2016 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Annex.GitOverlay where
+
+import qualified Control.Exception as E
+
+import Annex.Common
+import Git
+import Git.Types
+import Git.Env
+import qualified Annex
+
+{- Runs an action using a different git index file. -}
+withIndexFile :: FilePath -> Annex a -> Annex a
+withIndexFile f = withAltRepo
+	(\g -> addGitEnv g "GIT_INDEX_FILE" f)
+	(\g g' -> g' { gitEnv = gitEnv g })
+
+{- Runs an action using a different git work tree. -}
+withWorkTree :: FilePath -> Annex a -> Annex a
+withWorkTree d = withAltRepo
+	(\g -> return $ g { location = modlocation (location g) })
+	(\g g' -> g' { location = location g })
+  where
+	modlocation l@(Local {}) = l { worktree = Just d }
+	modlocation _ = error "withWorkTree of non-local git repo"
+
+{- Runs an action with the git index file and HEAD, and a few other
+ - files that are related to the work tree coming from an overlay
+ - directory other than the usual. This is done by pointing
+ - GIT_COMMON_DIR at the regular git directory, and GIT_DIR at the
+ - overlay directory. -}
+withWorkTreeRelated :: FilePath -> Annex a -> Annex a
+withWorkTreeRelated d = withAltRepo modrepo unmodrepo
+  where
+	modrepo g = do
+		let g' = g { location = modlocation (location g) }
+		addGitEnv g' "GIT_COMMON_DIR" =<< absPath (localGitDir g)
+	unmodrepo g g' = g' { gitEnv = gitEnv g, location = location g }
+	modlocation l@(Local {}) = l { gitdir = d }
+	modlocation _ = error "withWorkTreeRelated of non-local git repo"
+
+withAltRepo 
+	:: (Repo -> IO Repo)
+	-- ^ modify Repo
+	-> (Repo -> Repo -> Repo)
+	-- ^ undo modifications; first Repo is the original and second
+	-- is the one after running the action.
+	-> Annex a
+	-> Annex a
+withAltRepo modrepo unmodrepo a = do
+	g <- gitRepo
+	g' <- liftIO $ modrepo g
+	r <- tryNonAsync $ do
+		Annex.changeState $ \s -> s { Annex.repo = g' }
+		a
+	Annex.changeState $ \s -> s { Annex.repo = unmodrepo g (Annex.repo s) }
+	either E.throw return r