fix: fix handling of merge commits by using overlayworkingctx Most of this code was conceptually copied from what rebase does, with one small difference: hgext.rebaserev.rebase uses branchmerge=True, while I had to use branchmerge=False, or else it got really confused about updating to the same revision in some situations. I believe that the difference is that rebase is always dealing with *some* form of update - it never gets to mergemod.update if the source and destination are the same, while we can encounter that situation with fix. This may imply that this code has some issues with named branches that should be investigated. Differential Revision: https://phab.mercurial-scm.org/D7703