improve robustness of fromDirect and replaceFile

Made fromDirect check that a file in the tree has good content (and is not
a broken symlink either) before copying it to another file that has the
same key.

Made replaceFile clean up the temp file if the action that creates it, or
the file replacement action fails.

Remote/Git.hs

@@ -346,7 +346,7 @@ copyFromRemote' r key file dest
 			forever $
 				send =<< readSV v
 		let feeder = writeSV v . fromBytesProcessed
-		bracketIO noop (const $ tryIO $ killThread tid) (a feeder)
+		bracketIO noop (const $ tryIO $ killThread tid) (const $ a feeder)
 
 copyFromRemoteCheap :: Remote -> Key -> FilePath -> Annex Bool
 copyFromRemoteCheap r key file