Seems to be blocked by two safety checks and some to-be-determined semantics. [this is a dupe of http://www.bitbucket.org/durin42/hgsubversion/issue/27/revision-with-multiple-parents-cannot-be-pushed-caused-by-hg but it's got to be done]
- util.outgoing_revisions asserts that each revision only has one parent
- push_cmd.push_revisions_to_subversion bails out if the repo or any of the outgoing revisions have multiple parents. (The assertion in 1 should be preventing this from every being reached, before any risk of a partial commit.)
The obvious cases seem to have obvious semantics, but I'm not sure what edge cases to consider. Also, I don't know what information would need to be passed back for svn 1.5's merge tracking.