Take mergeinfo from union of svn:mergeinfo and svnmerge-integrated
The mergeinfo processing only looks at svnmerge-integrated if svn:mergeinfo is missing/empty.
We have a case in the GCC repository where svnmerge.py appears to have been used to merge from a branch with svn:mergeinfo to a branch that previously didn't have svn:mergeinfo, and that version of svnmerge.py didn't update svn:mergeinfo, just copied it. So the SVN merge, translated to git, only gets merge parents for the two branches mentioned in the copied svn:mergeinfo, and not for the /trunk:1-151687,151691-152437 specified in svnmerge-integrated (that is a contiguous range, there are no trunk commits in the gap) - but /trunk was actually the main source of the merge, so it should have got a merge parent for trunk and then optimized out the other two merge parents as being ancestors of that trunk commit.
I think the appropriate fix is to process both svn:mergeinfo and svnmerge-integrated, and, for each branch mentioned in either, take the union of the sets of revisions mentioned in the two properties. (/trunk is mentioned in both properties; its value in svn:mergeinfo is /trunk:130803-130804 which is a subset of the value in svnmerge-integrated.)