run-tests: add --with-chg option to run tests using chg Unlike --with-hg=/path/to/chg, this option allows us to start and clean up command servers in isolated environment. And we can specify the hg command as well as the chg command.

destutil: add more precise error classes for destmerge Having exception classes more precise than 'Abort' will allow us to properly catch "nothing to rebase" situations when we will be using 'destmerge' in rebase.

destutil: allow to disable the "on head check" in destmerge 'hg merge' refuses to pick a default destination if the working copy is not on a head. This is a very sensible default for 'hg merge' but 'hg rebase' should work in this situation. So we introduce a way to disable this check. It will soon be used by rebase.

destutil: allow to specify an explicit source for the merge We can now specify from where the merge is performed. The experimental revset is updated to take revisions as argument, allowing to test the feature. This will become very useful for pick the 'rebase' default destination. For this reason, we also exclude all descendants from the rebased set from the candidate destinations. This descendants exclusion was not necessary for merge as default destination would not be picked from anything else than a head. I'm not super excited with the current error messages, but I would prefer to delay an overall messages rework once 'hg rebase' is done getting a default destination aligned with 'hg merge'.

destutil: remove current head from list of candidates early While 'hg merge' will refuse to pick a default destination if the working copy is not on a head, this will be a common and valid case for rebase. In this case, we will need to exclude from the candidate destination all descendants from the rebased set. We make a step forward in that direction by removing parents of the working copy from the candidate destinations instead of manually filtering the working copy parent at the end of the process. This will make the extra step of filtering descendant much simpler in a future changeset.

destutil: add an 'action' layer to the destmerge message dictionary We'll introduce messages for 'rebase' soon, so we introduce a way to select a message based on the action being performed.

rebase: extract rebaseset and destination computation in a function The whole rebase function is gargantuan and this computation is almost 100 lines long. We extract it in a dedicated function as it is independent from the rest of the rebase code. Having it in its own function will make it easier to rework the default destination logic.

tests: have a more elaborated test for _destrebase The previous repo had a single lineage and a single head, pretty boring for rebase related testing.