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).