git-svn detects some merges incorrectly from svn:mergeinfo

Hi,

Now that git-svn detects merges from svn:mergeinfo, I thought it's time to re-import some of my SVN repos with git-svn. Detecting of the merges worked mostly well, but when inspecting the imported history with gitk, I found some incorrectly detected merges.

This patch series adds a test case and a fix for the case.

I was a bit uncertain if the fix should go to parents_exclude function or to "cater for merges which merge commits from multiple branches" section of find_extra_svn_parents function. I chose the latter and at least it does the trick.