destmerge: extract logic based on branch heads in its own function One of the main goal of having consolidated destination function is to allow extension to play with this logic. We extract sub logic to make is wrapping more practical.

destmerge: extract logic based on bookmark into its own function One of the main goal of having consolidated destination function is to allow extension to play with this logic. We extract sub logic to make is wrapping more practical.

destupdate: have a generic and extensible way to run each step We want extension to be able to easily override or add new way to select the default update destination. We use the same list + dict approach as in other parts of the code.

destupdate: extract logic based on branch in its own function One of the main goal of having consolidated destination function is to allow extension to play with this logic. We extract sub logic to make is wrapping more practical.