Switch to using relative paths to the git repository.

This allows the git repository to be moved while git-annex is running in
it, with fewer problems.

On Windows, this avoids some of the problems with the absurdly small
MAX_PATH of 260 bytes. In particular, git-annex repositories should
work in deeper/longer directory structures than before. See
http://git-annex.branchable.com/bugs/__34__git-annex:_direct:_1_failed__34___on_Windows/

There are several possible ways this change could break git-annex:

1. If it changes its working directory while it's running, that would
   be Bad News. Good news everyone! git-annex never does so. It would also
   break thread safety, so all such things were stomped out long ago.

2. parentDir "." -> "" which is not a valid path. I had to fix one
   instace of this, and I should probably wipe all calls to parentDir out
   of the git-annex code base; it was never a good idea.

3. Things like relPathDirToFile require absolute input paths,
   and code assumes that the git repo path is absolute and passes it to it
   as-is. In the case of relPathDirToFile, I converted it to not make
   this assumption.

Currently, the test suite has 16 failures.

Git.hs

@@ -31,6 +31,7 @@ module Git (
 	hookPath,
 	assertLocal,
 	adjustPath,
+	relPath,
 ) where
 
 import Network.URI (uriPath, uriScheme, unEscapeString)
@@ -141,25 +142,28 @@ hookPath script repo = do
 	isexecutable f = isExecutable . fileMode <$> getFileStatus f
 #endif
 
-{- Adusts the path to a local Repo.
- -
- - On windows, prefixing a path with \\?\ makes it be processed as a raw
- - path (/ is not converted to \). The benefit is that such a path does
- - avoids Windows's 260 byte limitation on the entire path. -}
-adjustPath :: Repo -> Repo
-adjustPath r@(Repo { location = l@(Local { gitdir = d, worktree = w }) }) = r
-	{ location = l 
-		{ gitdir = adjustPath' d
-		, worktree = fmap adjustPath' w
-		}
-	}
-adjustPath r@(Repo { location = LocalUnknown d }) = 
-	r { location = LocalUnknown (adjustPath' d) }
-adjustPath r = r
+{- Makes the path to a local Repo be relative to the cwd. -}
+relPath :: Repo -> IO Repo
+relPath = adjustPath torel
+  where
+	torel p = do
+		p' <- relPathCwdToFile p
+		if null p'
+			then return "."
+			else return p'
 
-adjustPath' :: FilePath -> FilePath
-#if mingw32_HOST_OS
-adjustPath' d = "\\\\?\\" ++ d
-#else
-adjustPath' = id
-#endif
+{- Adusts the path to a local Repo using the provided function. -}
+adjustPath :: (FilePath -> IO FilePath) -> Repo -> IO Repo
+adjustPath f r@(Repo { location = l@(Local { gitdir = d, worktree = w }) }) = do
+	d' <- f d
+	w' <- maybe (pure Nothing) (Just <$$> f) w
+	return $ r 
+		{ location = l 
+			{ gitdir = d'
+			, worktree = w'
+			}
+		}
+adjustPath f r@(Repo { location = LocalUnknown d }) = do
+	d' <- f d
+	return $ r { location = LocalUnknown d' }
+adjustPath _ r = pure r