test-editor-filename: fix portability of fake editor command

 - /bin/bash doesn't exist on FreeBSD
 - edit is executed by cmd.exe on Windows

tests/test-editor-filename.t

@@ -5,20 +5,18 b' Test temp file used with an editor has t'
 Create an editor that writes its arguments to stdout and set it to $HGEDITOR.
 
   $ cat > editor.sh << EOF
-  > #!/bin/bash
   > echo "\$@"
   > exit 1
   > EOF
-  $ chmod +x editor.sh
   $ hg add editor.sh
-  $ HGEDITOR=$TESTTMP/editor.sh
+  $ HGEDITOR="sh $TESTTMP/editor.sh"
   $ export HGEDITOR
 
 Verify that the path for a commit editor has the expected suffix.
 
   $ hg commit
   *.commit.hg.txt (glob)
-  abort: edit failed: editor.sh exited with status 1
+  abort: edit failed: sh exited with status 1
   [255]
 
 Verify that the path for a histedit editor has the expected suffix.
@@ -31,7 +29,7 b' Verify that the path for a histedit edit'
   $ hg commit --message 'At least one commit for histedit.'
   $ hg histedit
   *.histedit.hg.txt (glob)
-  abort: edit failed: editor.sh exited with status 1
+  abort: edit failed: sh exited with status 1
   [255]
 
 Verify that when performing an action that has the side-effect of creating an