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.

Annex.hs

@@ -187,7 +187,7 @@ newState c r = AnnexState
  - Ensures the config is read, if it was not already. -}
 new :: Git.Repo -> IO AnnexState
 new r = do
-	r' <- Git.adjustPath <$> Git.Config.read r
+	r' <- Git.Config.read =<< Git.relPath r
 	let c = extractGitConfig r'
 	newState c <$> if annexDirect c then fixupDirect r' else return r'