t6042: add testcase covering long chains of rename conflicts

t6042: add testcase covering long chains of rename conflicts

Each rename is a lego: the source side could be connected to a delete or
another rename, and the destination side could be connected to a rename or a
conflicting add.  Previous tests combined these to get e.g.
rename/rename(1to2)/add/add, rename/rename(2to1)/delete/delete, and
rename/add/delete.  But we can also build bigger chains of conflicts.  Add a
testcase demonstrating this.

Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>