SVN Mergeinfo properties on paths other than the working copy root
Solution 1:
Subversion 1.5.x adds a lot of svn:mergeinfo properties, even on files/folders which you think have nothing to do with the merge. But Subversion still uses those to reduce the merge time for subsequent merges.
If you don't like those, you can safely remove those modified/added svn:mergeinfo properties from all files/folders which were not part of the merge (leave it on the working copy root and the files/folders that got changes from the merge).
Subversion 1.6 will have those svn:mergeinfo properties reduced a lot if everything goes as planned with the 1.6 release.
Solution 2:
At least one part of this bug was fixed in Subversion 1.5.5. From the 1.5.5 CHANGES file:
do not create mergeinfo for wc-wc moves or copies (r34184, -585)
That is, there was a bug in SVN prior to 1.5 where it would create mergeinfo entries that it didn't use and were superfluous.