rewriteutil: fix broken touch of merge commits (issue6416)
`rewriteutil.rewrite()` is for rewriting a linear chain of commits into a single commit, i.e. what `hg fold` does. Many of the callers passed in a single commit because they wanted to rewrite just a single commit (e.g. `hg touch`). Before this patch, the code worked by going through the commits to fold and adding up all the modified files in them, then building a `memctx` based on that. As reported in issue6416, that can lose changes in merge commits. We could probably fix that without changing the existing code too much, but it seems the easiest way is to rewrite the code so it instead creates the new commit by effectively checking out the base and then revert to the head of the chain, so that's what this patch does. It does so by using in-memory merge.