add: Improved detection of files that are modified while being added.

In indirect mode, now checks the inode cache to detect changes to a file.
Note that a file can still be changed if a process has it open for write,
after landing in the annex.

In direct mode, some checking of the inode cache was done before, but
from a much later point, so fewer modifications could be detected. Now it's
as good as indirect mode.

On crippled filesystems, no lock down is done before starting to add a
file, so checking the inode cache is the only protection we have.

Command/AddUrl.hs

@@ -75,7 +75,11 @@ download url file = do
 	liftIO $ createDirectoryIfMissing True (parentDir tmp)
 	stopUnless (downloadUrl [url] tmp) $ do
 		backend <- chooseBackend file
-		let source = KeySource { keyFilename = file, contentLocation = tmp }
+		let source = KeySource
+			{ keyFilename = file
+			, contentLocation = tmp
+			, inodeCache = Nothing
+			}
 		k <- genKey source backend
 		case k of
 			Nothing -> stop