destutil: extract all 'mergedest' abort messages into a dictionary We plan to be able to reuse this function for rebase. The error message explicitly refers to "merge" in multiple places. So we'll need to be able to use different messages. The first step of that is to extract all messages in a dedicated dictionary and use them indirectly. As a side effect it clarifies the actual function and opens the way to various cleanups and fixes in future changesets.

tests: add an explicit destination in some rebase tests As we will make 'rebase' behave more like 'merge', it will no longer pick 'max(branch(.))' as the default destination. We have to hard code the expected destination is multiple tests where it matters. After a careful inspection none of theses tests really cares about the default destination behavior and just omitted one out of laziness.

tests: remove third head in some of the 'rebase-parameters' tests These tests do not care about that extra branch at all. In future changeset we will make rebase behave like merge and abort in case of an ambiguous destination (eg: multiple other heads) and that extra branch will make the command invocation breaks. We preventively remove this extra branch from the relevant tests in an independant changeset to reduce noise and increase confidence in the final change.