checkcopies: add an inline comment about the '_related' call This helps understanding the flow of the function.

checkcopies: minor change to comment This helped me understand the refactoring so this must be helpful.

checkcopies: rename 'ca' to 'base' This variable was named after the common ancestor. It is actually the merge base that might differ from the common ancestor in the graft case. We rename the variable before a larger refactoring to clarify the situation.

bisect: extra a small initialisation outside of a loop Having initialisation done during the first iteration is cute, but can be avoided.