t6423: add a testcase causing a failed assertion in process_renames

t6423: add a testcase causing a failed assertion in process_renames

If one side of history renames a directory A/ -> B/, and the other side
of history adds new files to A/, then directory rename detection notices
and moves or suggests moving those new files to B/.  A similar thing is
done for paths renamed into A/, causing them to be transitively renamed
into B/.  But, if the file originally came from B/, then this can end up
causing a file to be renamed back to itself.  merge-ort crashes under
this special case, due to a slightly overzealous assertion:

    git: merge-ort.c:3051: process_renames: Assertion `source_deleted || oldinfo->filemask & old_sidemask' failed.
    Aborted (core dumped)

Add a testcase demonstrating this.

Signed-off-by: Dmitry Goncharov <dgoncharov@users.sf.net>
[en: Instead of adding a new testsuite, place it near similar tests in
 t6423, adjusting to match the style of those tests.  Tweak the commit
 message to not repeat the entire testcase, but just describe the bug.
 Also update the line number in the error message.]
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>