If you speak of Mercurial's changesets and revisions, they are clearly not trivially computed from one another, being that a revision is clone-specific, and a c

If you speak of Mercurial's changesets and revisions, they are clearly not trivially computed from one another, being that a revision is clone-specific, and a changeset is a hash computed on a single file by looking at all it's ancestors' states. REPO1 and REPO2 can both have 5 revisions and only one is common to both — a merge is possible, but the changesets considered will be very different. If you're comparing SVN's revisions with Hg's changesets, still no dice, for the same reason: changesets are unique for a file, given that file's history, and the hash trail allows for a clear way to figure out where changes happened, and merge from that point. A SVN revision just tells you how many commits have happened up to that point.