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.

Command/Sync.hs

@@ -29,6 +29,7 @@ import qualified Remote.Git
 import Types.Key
 import Config
 import Annex.ReplaceFile
+import Git.FileMode
 
 import Data.Hash.MD5
 
@@ -321,7 +322,7 @@ resolveMerge' u
 		case msha of
 			Nothing -> a Nothing
 			Just sha -> do
-				key <- catKey sha
+				key <- catKey sha symLinkMode
 				maybe (return False) (a . Just) key
 
 {- The filename to use when resolving a conflicted merge of a file,