annex.thin

Decided it's too scary to make v6 unlocked files have 1 copy by default,
but that should be available to those who need it. This is consistent with
git-annex not dropping unused content without --force, etc.

* Added annex.thin setting, which makes unlocked files in v6 repositories
  be hard linked to their content, instead of a copy. This saves disk
  space but means any modification of an unlocked file will lose the local
  (and possibly only) copy of the old version.
* Enable annex.thin by default on upgrade from direct mode to v6, since
  direct mode made the same tradeoff.
* fix: Adjusts unlocked files as configured by annex.thin.

Command/Lock.hs

@@ -79,7 +79,7 @@ performNew file key filemodified = do
 		unlessM (sameInodeCache obj (maybeToList mfc)) $ do
 			modifyContent obj $ replaceFile obj $ \tmp -> do
 				unlessM (checkedCopyFile key obj tmp) $
-					error "unable to lock file; need more free disk space"
+					error "unable to lock file"
 			Database.Keys.storeInodeCaches key [obj]
 
 	-- Try to repopulate obj from an unmodified associated file.