This patch adds `jj obslog -p` for including the diff compared to the
predecessor (the first predecessor if there are several). If the
predecessor's parents are different, then we create a temporary tree
by rebasing the predecessor to have the same parents and we use the
result as base for the diff. That way, we avoid polluting the diff
with the changes caused by the rebase. (I don't think we currently
have any commands that can change both parents and content, so the
diff should always be empty for rewrites caused by a rebase.)
Working on this also reminded me that it'll be really nice when we
replace `jj obslog` by something based on the operation log - I really
miss seeing information about the operation in the output (like `hg
obslog` gets from its obsmarkers).
