It's currently possible to have patches with multiple old path name
headers. As we didn't check for this case, this resulted in a memory
leak when overwriting the old old path with the new old path because we
simply discarded the old pointer.

Instead of fixing this by free'ing the old pointer, we should reject
such patches altogether. It doesn't make any sense for the "---" or
"+++" markers to occur multiple times within a patch n the first place.
This also implicitly fixes the memory leak.