pre-commit: Update metadata when committing changes to annexed files within a view.

So the user can now switch to a view and then move files around within it
to manage metadata. For example, moving a file into a new directory
when in the tags=* view adds a tag to it.

Implementation is fairly efficient. One diff-index, which is no more
expensive than the first stage of a git commit, followed by possibly
some cat-file --batch traffic to find the key (when deleting a file).
Very similar to what's done in direct mode when committing. And like
direct mode when updating the WC after a merge, it has to buffer the
diff-tree values in order to make 2 passes over them.

When not in a view, pre-commit now does one extra git symbolic-ref,
which is tiny overhead.

This commit was sponsored by Andrew Eskridge.

Logs/MetaData.hs

@@ -28,8 +28,6 @@
 module Logs.MetaData (
 	getCurrentMetaData,
 	getMetaData,
-	setMetaData,
-	unsetMetaData,
 	addMetaData,
 	addMetaData',
 	currentMetaData,
@@ -58,16 +56,6 @@ getCurrentMetaData = currentMetaData . collect <$$> getMetaData
   where
 	collect = foldl' unionMetaData newMetaData . map value . S.toAscList
 
-setMetaData :: Key -> MetaField -> String -> Annex ()
-setMetaData = setMetaData' True
-
-unsetMetaData :: Key -> MetaField -> String -> Annex ()
-unsetMetaData = setMetaData' False
-
-setMetaData' :: Bool -> Key -> MetaField -> String -> Annex ()
-setMetaData' isset k field s = addMetaData k $
-	updateMetaData field (mkMetaValue (CurrentlySet isset) s) newMetaData
-
 {- Adds in some metadata, which can override existing values, or unset
  - them, but otherwise leaves any existing metadata as-is. -}
 addMetaData :: Key -> MetaData -> Annex ()