transaction: change list of journal entries into a dictionary The transaction object used to keep a mapping table of path names to journal entries and a list of journal entries consisting of path and file offset to truncate on rollback. The offsets are used in three cases. repair.strip and rollback process all of them in one go, but they care about the order. For them, it is perfectly reasonable to read the journal back from disk as both operations already involve at least one system call per journal entry. The other consumer is the revlog logic for moving from inline to external data storage. It doesn't care about the order of the journal and just needs to original offset stored. Further optimisations are possible here to move the in-memory journal to a set(), but without memoisation of the original revlog size this could turn it into O(n^2) behavior in worst case when many revlogs need to migrated. Differential Revision: https://phab.mercurial-scm.org/D9277