test-merge-prompt.t: unset HGMERGE

We're going to test behavior with and without HGMERGE, so disable it for the
test.

tests/test-merge-prompt.t

@@ -5,6 +5,9 b' b5605d88dc27: Make ui.prompt repeat on "'
 840e2b315c1f: Fix misleading error and prompts during update/merge
  (issue556)
 
+Make sure HGMERGE doesn't interfere with the test
+  $ unset HGMERGE
+
   $ status() {
   >     echo "--- status ---"
   >     hg st -A file1 file2