histedit: fix test-histedit-edit on vfat

test-histedit-edit was broken because it relied on the HGEDITOR script being
executable. Instead, lets just execute 'sh' and pass it the script to run. This
seems to be the pattern followed in other tests.

tests/test-histedit-edit.t

@@ -71,15 +71,14 b' edit the history'
   When you are finished, run hg histedit --continue to resume.
 
 edit the plan via the editor
-  $ cat >> ../editplan.sh <<EOF
+  $ cat >> $TESTTMP/editplan.sh <<EOF
   > cat > \$1 <<EOF2
   > drop e860deea161a e
   > drop 652413bf663e f
   > drop 3c6a8ed2ebe8 g
   > EOF2
   > EOF
-  $ chmod a+x ../editplan.sh
-  $ HGEDITOR=../editplan.sh hg histedit --edit-plan
+  $ HGEDITOR="sh $TESTTMP/editplan.sh" hg histedit --edit-plan
   $ cat .hg/histedit-state
   v1
   055a42cdd88768532f9cf79daa407fc8d138de9b