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.
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null