histedit: use "getcommiteditor()" instead of explicit editor choice for "--continue"
This omits (redundant) adding "\n' to "message", because:
- empty line is inserted by "commitforceeditor", if editor is invoked
- tail white-spaces are stripped at storing into chaneglog, otherwise
This patch also enhances "test-histedit-edit.t", because "hg histedit"
hasn't been explicitly tested around editor invocation and
"--continue" option.