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'.