rebase: simplify documentation about selecting commits to rebase

rebase: simplify documentation about public commits add reference to graft

verify: clean up weird error/warning lists Nested functions in Python are not able to assign to variables in the outer scope without something like the list trick because assignments refer to the inner scope. So, we formerly used a list to give an object to assign into. Now that error and warning are object members, the [0] hack is no longer needed.

diff: clarify comparison as first parent