fold: allow operations on merge commits with some conditions It's possible to fold revision chains that include a single merge commit: just fold everything into the merge commit while saving its other parent (so it continues being a merge commit). It's also possible to fold revisions that include multiple merge commits, on the condition that they merge with not more than 2 external changesets (i.e. a changesets that aren't going to be folded).

rewind: make sure merge commits include files from p1 and p2 Otherwise rewinding a merge commit makes it lose all changes. This fix populates `updates` argument of rewriteutil.rewrite() with parent changesets. That argument is normally used for folding multiple changesets, but in this case it's simply used to include files from p1 and p2. Usually, rewrite() works fine using ctx.files(), but that function can return an empty list when ctx is a merge commit.