See http://www.guiffy.com/SureMergeWP.html for another merge test suite with some background material. A year ago i tried a few of them in various diff-tools, none passed all of the tests, including guiffy even though they claim to in the article. Some of the tests can also be considered objective or non-resolvable but it still an eye opener to see how poor the merge tools really are.
Btw, i thought merge conflict handling was a feature of the diff tool, not the scm?
Try Beyond Compare (I'm not affiliated, just a longtime customer). I just went through each test case and had no problems. Resolving one of the standard conflicts involved using 'align with <F7>' to separate the changes at the end of the file. Most of the pathological cases were solved automatically and without even conflict markers, and when they weren't, selecting a hunk, right clicking, and choosing 'take left then right' worked.
When a diff tool is included with the scm, it's hard for the average user to separate the functionality even if it's possible.
And odds are you'll probably to be able to use different diff/merge tools to handle various file formats (plain text, xml, binary, etc.) so you won't have to rely on just one getting everything right.
Btw, i thought merge conflict handling was a feature of the diff tool, not the scm?