evolve: create new commit while resolving content-divergence
Before this patch, we used to amend one of the content-divergent changes with
the changes from the merge. Creating a new commit will certain advantages like
letting user enter a commit message, showing user the new merge diff before
commiting the content-divergent resolution and give us more control in the
implementation part.
The reason amend was used initially was just because it was easy. There are some
tests update due to change.