t6036, t6042: testcases for rename collision of already conflicting files

t6036, t6042: testcases for rename collision of already conflicting files

When a single file is renamed, it can also be modified, yielding the
possibility of that renamed file having content conflicts.  If two
different such files are renamed into the same location, then two-way
merging those files may result in nested conflicts.  Add a testcase that
makes sure we get this case correct, and uses different lengths of
conflict markers to differentiate between the different nestings.

Also add another case with an extra (i.e. third) level of conflict
markers due to using merge.conflictstyle=diff3 and the virtual merge
base also having conflicts present.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>