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.

mercurial/context.py

@@ -208,9 +208,7 b' def makememctx(repo, parents, text, user'
     if branch:
         extra['branch'] = encoding.fromlocal(branch)
     ctx =  memctx(repo, parents, text, files, getfilectx, user,
-                          date, extra)
-    if editor:
-        ctx._text = editor(repo, ctx, [])
+                          date, extra, editor)
     return ctx
 
 class changectx(basectx):
@@ -1287,7 +1285,7 b' class memctx(object):'
     is a dictionary of metadata or is left empty.
     """
     def __init__(self, repo, parents, text, files, filectxfn, user=None,
-                 date=None, extra=None):
+                 date=None, extra=None, editor=False):
         self._repo = repo
         self._rev = None
         self._node = None
@@ -1305,6 +1303,10 b' class memctx(object):'
         if self._extra.get('branch', '') == '':
             self._extra['branch'] = 'default'
 
+        if editor:
+            self._text = editor(self._repo, self, [])
+            self._repo.savecommitmessage(self._text)
+
     def __str__(self):
         return str(self._parents[0]) + "+"