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/Migrate.hs

@@ -63,5 +63,9 @@ perform file oldkey oldbackend newbackend = do
 		next $ Command.ReKey.cleanup file oldkey newkey
 	genkey = do
 		content <- inRepo $ gitAnnexLocation oldkey
-		let source = KeySource { keyFilename = file, contentLocation = content }
+		let source = KeySource
+			{ keyFilename = file
+			, contentLocation = content
+			, inodeCache = Nothing
+			}
 		liftM fst <$> genKey source (Just newbackend)