optimise fsck --from normal git remotes

For a local git remote, can symlink the file.
For a git remote using rsync, can preseed any local content.

There are a few reasons to use fsck --from on a normal git remote.
One is if it's using gitosis or similar, and you don't have shell access
to run git annex locally. Another reason could be if you just want to
fsck certian files of a bare remote.

Annex/Content.hs

@@ -22,6 +22,7 @@ module Annex.Content (
 	getKeysPresent,
 	saveState,
 	downloadUrl,
+	preseedTmp,
 ) where
 
 import System.IO.Error (try)
@@ -40,6 +41,7 @@ import Utility.FileMode
 import qualified Utility.Url as Url
 import Types.Key
 import Utility.DataUnits
+import Utility.CopyFile
 import Config
 import Annex.Exception
 
@@ -301,3 +303,12 @@ downloadUrl urls file = do
 	g <- gitRepo
 	o <- map Param . words <$> getConfig g "web-options" ""
 	liftIO $ anyM (\u -> Url.download u o file) urls
+
+{- Copies a key's content, when present, to a temp file.
+ - This is used to speed up some rsyncs. -}
+preseedTmp :: Key -> FilePath -> Annex ()
+preseedTmp key file =
+	unlessM (liftIO $ doesFileExist file) $ whenM (inAnnex key) $ do
+		s <- inRepo $ gitAnnexLocation key
+		liftIO $ whenM (copyFileExternal s file) $
+			allowWrite file