evolve: fix `hg abort` when hg next --evolve leads to conflicts (issue5897)
Before this patch, if you do `hg next --evolve` and end up having conflicts and
then you try `hg abort`, it will throw a KeyError. This patch fixes that.
tests: add a test demonstrating issue5897
When conflicts are caused due to `hg next --evolve`, `hg evolve --abort` breaks!
This patch adds test to demonstrate that. This will be fixed in next patch.