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.

Annex/Direct.hs

@@ -79,6 +79,7 @@ addDirect file cache = do
 	let source = KeySource
 		{ keyFilename = file
 		, contentLocation = file
+		, inodeCache = Just cache
 		}
 	got =<< genKey source =<< chooseBackend file
   where