backout: avoid update on simple case. Before the changeset the backout process was: 1) go to <target> 2) revert to <target> parent 3) update back to changeset we came from The two update steps can takes a very long time to move back and forth unrelated file change between <target> and current working directory. The new process is just merging current working directory with the parent of <target> using <target> as ancestor. This give the very same result but skip the two updates. On big repo with a lot of files and changes that save a lots of time (x20 for one week window). The "merge" version (hg backout --merge) is still done with upgrades. We could imagine using in memory commit to speed it up but this is another fish.