more completely solve catKey memory leak

Done using a mode witness, which ensures it's fixed everywhere.

Fixing catFileKey was a bear, because git cat-file does not provide a
nice way to query for the mode of a file and there is no other efficient
way to do it. Oh, for libgit2..

Note that I am looking at tree objects from HEAD, rather than the index.
Because I cat-file cannot show a tree object for the index.
So this fix is technically incomplete. The only cases where it matters
are:

1. A new large file has been directly staged in git, but not committed.
2. A file that was committed to HEAD as a symlink has been staged
   directly in the index.

This could be fixed a lot better using libgit2.

Git/CatFile.hs

@@ -130,4 +130,4 @@ catTree h treeref = go <$> catObjectDetails h treeref
 	parsemodefile b = 
 		let (modestr, file) = separate (== ' ') (encodeW8 $ L.unpack b)
 		in (file, readmode modestr)
-	readmode = fst . Prelude.head . readOct
+	readmode = fst . fromMaybe (0, undefined) . headMaybe . readOct