rebase: use "getcommiteditor()" instead of explicit editor choice This patch also enhances "test-rebase-scenario-global.t", because "hg rebase" hasn't been explicitly tested around editor invocation and "--edit" option. In the other hand, this patch doesn't enhance tests in "hg rebase --collapse" case, because it is already tested in "test-rebase-collapse.t".

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.

histedit: use "getcommiteditor()" instead of explicit editor choice for "fold" This patch doesn't change any tests like as preceding patches, because editor invocation is already tested in "test-histedit-fold.t".