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.