Properly copy mergeinfo ranges when there is a gap.

The existing logic failed to produce a complete union of mergeinfo ranges in cases where there were both gaps and overlaps, resulting in a suboptimal set of merge parents for GCC commit 152464. The svnmerge-integrated property in the included test illustrates the problem, although it's only like to appear in practice when merge information from svnmerge-integrated and svn:mergeinfo is combined.