[MERGE] add stop_rule to Branch.iter_merge_sorted_revisions()

[Ian Clatworthy]

Martin Albisetti wrote:

> That said, what would be the main difference between include and with-merges?

include terminates the iteration immediately after finding stop-rev-id in the
ordered list of (rev-id, revno, depth) tuples. (log --short behaviour.)

with-merges continues iterating beyond that if and only if the subsequent
tuples are deeper. (log --long behaviour.)

Ian C.