histedit: use "getcommiteditor()" instead of explicit editor choice for "fold"

This patch doesn't change any tests like as preceding patches, because
editor invocation is already tested in "test-histedit-fold.t".

hgext/histedit.py

@@ -299,7 +299,7 b' def collapse(repo, first, last, commitop'
                          user=user,
                          date=date,
                          extra=extra,
-                         editor=cmdutil.commitforceeditor)
+                         editor=cmdutil.getcommiteditor(edit=True))
     return repo.commitctx(new)
 
 def pick(ui, repo, ctx, ha, opts):