work around git bug

Work around git bug that runs smudge/clean filters at the top of the
repository while passing them a relative GIT_WORK_TREE that may point
outside of the repository, by using GIT_PREFIX to get back to the
subdirectory where a relative GIT_WORK_TREE is valid.

git devs have been informed of the bug and may fix it, which could conveivably
break this fix, but as it is, this works back to git 1.7.6.

This commit was sponsored by Jochen Bartl on Patreon.

Git/CurrentRepo.hs

@@ -25,12 +25,20 @@ import Utility.Env.Set
  - directory if necessary to ensure it is within the repository's work
  - tree. While not needed for git commands, this is useful for anything
  - else that looks for files in the worktree.
+ -
+ - Also works around a git bug when running some hooks. It
+ - runs the hooks in the top of the repository, but if GIT_WORK_TREE
+ - was relative, it then points to the wrong directory. In this situation
+ - GIT_PREFIX contains the directory that GIT_WORK_TREE (and GIT_DIR)
+ - are relative to.
  -}
 get :: IO Repo
 get = do
-	gd <- pathenv "GIT_DIR"
+	prefix <- getpathenv "GIT_PREFIX"
+	gd <- pathenv "GIT_DIR" prefix
 	r <- configure gd =<< fromCwd
-	wt <- maybe (worktree $ location r) Just <$> pathenv "GIT_WORK_TREE"
+	wt <- maybe (worktree $ location r) Just
+		<$> pathenv "GIT_WORK_TREE" prefix
 	case wt of
 		Nothing -> return r
 		Just d -> do
@@ -39,13 +47,18 @@ get = do
 				setCurrentDirectory d
 			return $ addworktree wt r
   where
-	pathenv s = do
+	getpathenv s = do
 		v <- getEnv s
 		case v of
 			Just d -> do
 				unsetEnv s
-				Just <$> absPath d
+				return (Just d)
 			Nothing -> return Nothing
+	
+	pathenv s Nothing = getpathenv s
+	pathenv s (Just prefix) = getpathenv s >>= \case
+		Nothing -> return Nothing
+		Just d -> Just <$> absPath (prefix </> d)
 
 	configure Nothing (Just r) = Git.Config.read r
 	configure (Just d) _ = do