evolve: prompt user for branch selection while resolving content-divergence When resolving content-divergence, there can be case when both the content-divergent changesets are on different named branches. In that case, we are not sure which branch should the resolution commit should be on. This patch adds logic to prompt user to choose the branch which the resolution commit should be on and then creates the new resolution commit on that branch. This also adds a new test file which have tests for the branch selection feature we have added.

evolve: pass the base revision in _completecontentdivergent() We need the base revision to merge commit messages, branches and more stuff. Let's make sure we have that by passing into the function which completes the divergence resolution.

evolve: store the base of content-divergents in evolvestate We need the base changeset for merging commit messages, branches. Let's store it in evolvestate.