histedit: pass "editor" argument to "memctx.__init__()" for "collapse" command

The preceding patch causes that "memctx.__init__()" with "editor"
argument invokes editor and saves edited commit message into
".hg/last-message.txt".

This patch passes "editor" argument to "memctx.__init__()" instead of
explicit invocations of "commitforceeditor()" and
"savecommitmessage()" for "collapse" command.

hgext/histedit.py

@@ -298,9 +298,8 b' def collapse(repo, first, last, commitop'
                          filectxfn=filectxfn,
                          user=user,
                          date=date,
-                         extra=extra)
-    new._text = cmdutil.commitforceeditor(repo, new, [])
-    repo.savecommitmessage(new.description())
+                         extra=extra,
+                         editor=cmdutil.commitforceeditor)
     return repo.commitctx(new)
 
 def pick(ui, repo, ctx, ha, opts):