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.

--- a/tests/test-backout.t	Thu Jan 16 12:08:57 2014 +0100
+++ b/tests/test-backout.t	Wed Jan 08 14:53:46 2014 -0800
@@ -188,7 +188,6 @@
 
 without --merge
   $ hg backout -d '3 0' 1 --tool=true
-  reverting a
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg locate b
   b
@@ -324,8 +323,7 @@
 
 without --merge
   $ hg backout -r 1 --tool=true
-  removing file1
-  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
   $ hg branch
   branch2
   $ hg status -A