Currently, backout is creating a backout revision as a child node of the
backed out node and will leave you at this new head. This has several
drawbacks:
* this changes the current head
* when there is a long history between the backed out node and the
current head, this will generate a huge number of diffs that are scary
at first sight, and not very natural to review before commit.
The change consists to switch back to the original node as soon as the
backout node (which becomes the new tip) has been created. Then the
--merge option can just merge this new tip in the current node.
* the current head/node is not changed from the user's point of view
* even without using the --merge option, the backout revision is still
easy to locate, as this is the tip
* the merge is much more intuitive as diffs of the merge is right you
are looking to backout
