t4301: add more interesting merge-tree testcases

t4301: add more interesting merge-tree testcases

This adds several tests of `merge-tree -z` extended conflict output
behavior to the testsuite, including some tests adapted from t6422.
These tests mark current behavior, not necessarily optimal behavior.  In
particular, some path_msg() calls might want to include additional
paths.

These testcases also make something clear about the <Conflicted file>
info section of the output.  That section consists of a sequence of
lines of the form
    <mode> <object> <stage> <filename>
where <stage> is always greater than 0 (since each line comes from a
conflicted file).  The lines correspond to conflicts that would be
placed in the index if we were doing a merge in a working tree.  It is
perhaps natural to assume that for any given line, the <object> and
<filename> correspond to a single <revision>:<filename> pair from one of
the commits being merged (or from the merge base).  This is true for
simple conflicts.  However, these testcases make it clear that this is
not the case in general.  For example, <object> may be the hash of a
three-way content merge of three different files (and with different
filenames).

The tests no longer pass under TEST_PASSES_SANITIZE_LEAK; it appears
that doing a directory rename with "git mv", among other possible
problems, triggers issues.

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