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/CheckAttr.hs

@@ -84,7 +84,7 @@ checkAttr (h, attrs, oldgit, currdir) want file = do
 	 - so use relative filenames. -}
 	file'
 		| oldgit = absPathFrom currdir file
-		| otherwise = relPathDirToFile currdir $ absPathFrom currdir file
+		| otherwise = relPathDirToFileAbs currdir $ absPathFrom currdir file
 	oldattrvalue attr l = end bits !! 0
 	  where
 		bits = split sep l