backout: avoid update on simple case....
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.
