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?

Locations.hs

@@ -236,11 +236,14 @@ gitAnnexRemotesDir r = addTrailingPathSeparator $ gitAnnexDir r </> "remotes"
 gitAnnexAssistantDefaultDir :: FilePath
 gitAnnexAssistantDefaultDir = "annex"
 
-{- Checks a symlink target to see if it appears to point to annexed content. -}
+{- Checks a symlink target to see if it appears to point to annexed content.
+ -
+ - We only look at paths inside the .git directory, and not at the .git
+ - directory itself, because GIT_DIR may cause a directory name other
+ - than .git to be used.
+ -}
 isLinkToAnnex :: FilePath -> Bool
-isLinkToAnnex s = ('/':d) `isInfixOf` s || d `isPrefixOf` s
-  where
-	d = ".git" </> objectDir
+isLinkToAnnex s = ('/':objectDir) `isInfixOf` s
 
 {- Converts a key into a filename fragment without any directory.
  -