histedit: replaces patching logic by merges
The old and fragile patching logic is replaced by smart merges (as rebase and
graft do). This should prevents some conflicts and smoother human resolution.
For this purpose the "foldchanges" function is renamed to "applychanges" and
handle a single revision only.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.