When accessing a local remote, shut down git-cat-file processes afterwards, to ensure that remotes on removable media can be unmounted. Closes: #758630

This does mean that eg, copying multiple files to a local remote will
become slightly slower, since it now restarts git-cat-file after each copy.
Should not be significant slowdown.

The reason git-cat-file is run on the remote at all is to update its
location log. In order to add an item to it, it needs to get the current
content of the log. Finding a way to avoid needing to do that would be a
good path to avoiding this slowdown if it does become a problem somehow.

This commit was sponsored by Evan Deaubl.

Annex.hs

@@ -15,6 +15,7 @@ module Annex (
 	eval,
 	getState,
 	changeState,
+	withState,
 	setFlag,
 	setField,
 	setOutput,
@@ -214,6 +215,11 @@ changeState modifier = do
 	mvar <- ask
 	liftIO $ modifyMVar_ mvar $ return . modifier
 
+withState :: (AnnexState -> (AnnexState, b)) -> Annex b
+withState modifier = do
+	mvar <- ask
+	liftIO $ modifyMVar mvar $ return . modifier
+
 {- Sets a flag to True -}
 setFlag :: String -> Annex ()
 setFlag flag = changeState $ \s ->