Additional GIT_DIR support bugfixes. May actually work now.

Two fixes. First, and most importantly, relax the isLinkToAnnex check
to only look for /annex/objects/, not [^|/].git/annex/objects. If
GIT_DIR is used with a detached work tree, the git directory is
not necessarily named .git.

There are important caveats with doing that at all, since git-annex will
make symlinks that point at GIT_DIR, which means that the relative path
between GIT_DIR and GIT_WORK_TREE needs to remain stable across all clones
of the repository.

----

The other fix is just fixing crazy and wrong code that, when GIT_DIR is
set, expects to still find a git repository in the path below the work
tree, and uses some of its configuration, and some of GIT_DIR. What was I
thinking, and why can't I seem to get this code right?

Git/Construct.hs

@@ -17,6 +17,7 @@ module Git.Construct (
 	fromRemotes,
 	fromRemoteLocation,
 	repoAbsPath,
+	newFrom,
 ) where
 
 import System.Posix.User
@@ -31,17 +32,16 @@ import Utility.UserInfo
 
 {- Finds the git repository used for the cwd, which may be in a parent
  - directory. -}
-fromCwd :: IO Repo
-fromCwd = getCurrentDirectory >>= seekUp checkForRepo
+fromCwd :: IO (Maybe Repo)
+fromCwd = getCurrentDirectory >>= seekUp
   where
-	norepo = error "Not in a git repository."
-	seekUp check dir = do
-		r <- check dir
+	seekUp dir = do
+		r <- checkForRepo dir
 		case r of
 			Nothing -> case parentDir dir of
-				"" -> norepo
-				d -> seekUp check d
-			Just loc -> newFrom loc
+				"" -> return Nothing
+				d -> seekUp d
+			Just loc -> Just <$> newFrom loc
 
 {- Local Repo constructor, accepts a relative or absolute path. -}
 fromPath :: FilePath -> IO Repo