fileRef: make paths relative and simplified

Fix behavior of several commands, including reinject, addurl, and rmurl
when given an absolute path to an unlocked file, or a relative path that
leaves and re-enters the repository.

To avoid slowing down all the cases where the paths are already ok
with an unncessary call to getCurrentDirectory, put in an optimisation
in relPathCwdToFile. That will probably also speed up other parts of
git-annex by some small amount, but I have not benchmarked.

Note that I did not convert branchFileRef, because it seems likely that
it will be used with a file that is not provided by the user, so is already
in a sane format. This is certainly true for the way git-annex uses it,
though maybe arguable to the extent Git.Ref is a reusable library.

Annex/CatFile.hs

@@ -178,11 +178,11 @@ catSymLinkTarget sha = fromInternalGitPath . L.toStrict <$> get
 catKeyFile :: RawFilePath -> Annex (Maybe Key)
 catKeyFile f = ifM (Annex.getState Annex.daemon)
 	( catKeyFileHEAD f
-	, catKey $ Git.Ref.fileRef f
+	, catKey =<< liftIO (Git.Ref.fileRef f)
 	)
 
 catKeyFileHEAD :: RawFilePath -> Annex (Maybe Key)
-catKeyFileHEAD f = catKey $ Git.Ref.fileFromRef Git.Ref.headRef f
+catKeyFileHEAD f = catKey =<< liftIO (Git.Ref.fileFromRef Git.Ref.headRef f)
 
 {- Look in the original branch from whence an adjusted branch is based
  - to find the file. But only when the adjustment hides some files. -}
@@ -194,5 +194,6 @@ catObjectMetaDataHidden = hiddenCat catObjectMetaData
 
 hiddenCat :: (Ref -> Annex (Maybe a)) -> RawFilePath -> CurrBranch -> Annex (Maybe a)
 hiddenCat a f (Just origbranch, Just adj)
-	| adjustmentHidesFiles adj = a (Git.Ref.fileFromRef origbranch f)
+	| adjustmentHidesFiles adj = 
+		a =<< liftIO (Git.Ref.fileFromRef origbranch f)
 hiddenCat _ _ _ = return Nothing