cmdutil: introduce "getcommiteditor()" to simplify code paths to choose editor

"getcommiteditor()" can simplify code paths to choose commit editor
according to '--edit' option as below:

    before:
        editor = cmdutil.commiteditor # or editor = None/False
        if opts.get('edit'):
            editor = cmdutil.commitforceeditor

    after:
        editor = cmdutil.getcommiteditor(**opts)

"getcommiteditor()" accepts option arguments not in "opts" style but
in "**opts" style, because some code paths want to invoke it with just
explicit "edit=True" argument (building dictionary is redundant).

mercurial/cmdutil.py

@@ -109,6 +109,13 b' def logmessage(ui, opts):'
                              (logfile, inst.strerror))
     return message
 
+def getcommiteditor(edit=False, **opts):
+    """get appropriate commit message editor according to '--edit' option"""
+    if edit:
+        return commitforceeditor
+    else:
+        return commiteditor
+
 def loglimit(opts):
     """get the log limit according to option -l/--limit"""
     limit = opts.get('limit')