git's handing of relative GIT_INDEX_FILE is more insane than I thought; always make absolute

This is actually worse than I thought; when git is being run with a
detached work tree, GIT_INDEX_FILE is treated as a path relative to CWD,
instead of the normal behavior of relative the top of the work tree.

This seems to make it basically impossible for any program that wants to
use GIT_INDEX_FILE to use anything other than an absolute path to it; there
are too many configurations to keep straight that can change how git
interprets what should be a simple relative path to a file.

(I have complained to the git developers.)

Annex/GitOverlay.hs

@@ -19,7 +19,7 @@ import qualified Annex
 {- Runs an action using a different git index file. -}
 withIndexFile :: FilePath -> Annex a -> Annex a
 withIndexFile f a = do
-	f' <- inRepo $ indexEnvVal f
+	f' <- liftIO $ indexEnvVal f
 	withAltRepo
 		(\g -> addGitEnv g indexEnv f')
 		(\g g' -> g' { gitEnv = gitEnv g })