context: move editor invocation from "makememctx()" to "memctx.__init__()" This patch introduces "editor" argument to "memctx.__init__()", and moves editor invocation from "makememctx()" to "memctx.__init__()", to centralize editor invocation into "memctx" object creation. This relocation is needed, because "makememctx()" requires the "store" object providing "getfile()" to create "memfilectx" object, and this prevents some code paths from using "makememctx()" instead of "memctx.__init__()". This patch also invokes "localrepository.savecommitmessage()", when "editor" is specified explicitly, to centralize saving commit message into "memctx" object creation: passing "cmdutil.commiteditor" as "editor" can achieve both suppressing editor invocation and saving into ".hg/last-message.txt" for non empty commit messages.