merge-recursive: adjust git-ls-tree use for the latest.

You need to pass -t flag if you want to see tree objects in
"git-ls-tree -r" output these days.  This change broke the tree
structure reading code in git-merge-recursive used to detect D/F
conflicts.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-merge-recursive.py

@@ -98,7 +98,7 @@ def merge(h1, h2, branch1Name, branch2Name, graph, callDepth=0):
 def getFilesAndDirs(tree):
     files = Set()
     dirs = Set()
-    out = runProgram(['git-ls-tree', '-r', '-z', tree])
+    out = runProgram(['git-ls-tree', '-r', '-z', '-t', tree])
     for l in out.split('\0'):
         m = getFilesRE.match(l)
         if m: