merge-recursive: handle rename-to-self case

Directory rename detection can cause transitive renames, e.g. if the two different sides of history each do one half of: A/file -> B/file B/ -> C/ then directory rename detection transitively renames to give us A/file -> C/file However, when C/ == A/, note that this gives us A/file -> A/file. merge-recursive assumed that any rename D -> E would have D != E. While that is almost always true, the above is a special case where it is not. So we cannot do things like delete the rename source, we cannot assume that a file existing at path E implies a rename/add conflict and we have to be careful about what stages end up in the output. This change feels a bit hackish. It took me surprisingly many hours to find, and given merge-recursive's design causing it to attempt to enumerate all combinations of edge and corner cases with special code for each combination, I'm worried there are other similar fixes needed elsewhere if we can just come up with the right special testcase. Perhaps an audit would rule it out, but I have not the energy. merge-recursive deserves to die, and since it is on its way out anyway, fixing this particular bug narrowly will have to be good enough. Reported-by: Anders Kaseorg <andersk@mit.edu> Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>