Collect merge-related tests to t64xx

The tests for the merge machinery are spread over several places. Collect them into t64xx for simplicity. Some notes: t60[234]*.sh: Merge tests started in t602*, overgrew bisect and remote tracking tests in t6030, t6040, and t6041, and nearly overtook replace tests in t6050. This made picking out relevant tests that I wanted to run in a tighter loop slightly more annoying for years. t303*.sh: These started out as tests for the 'merge-recursive' toplevel command, but did not restrict to that and had lots of overlap with the underlying merge machinery. t7405, t7613: submodule-specific merge logic started out in submodule.c but was moved to merge-recursive.c in commit 18cfc08866 ("submodule.c: move submodule merging to merge-recursive.c", 2018-05-15). Since these tests are about the logic found in the merge machinery, moving these tests to be with the merge tests makes sense. t7607, t7609: Having tests spread all over the place makes it more likely that additional tests related to a certain piece of logic grow in all those other places. Much like t303*.sh, these two tests were about the underlying merge machinery rather than outer levels. Tests that were NOT moved: t76[01]*.sh: Other than the four tests mentioned above, the remaining tests in t76[01]*.sh are related to non-recursive merge strategies, parameter parsing, and other stuff associated with the highlevel builtin/merge.c rather than the recursive merge machinery. t3[45]*.sh: The rebase testcases in t34*.sh also test the merge logic pretty heavily; sometimes changes I make only trigger failures in the rebase tests. The rebase tests are already nicely coupled together, though, and I didn't want to mess that up. Similar comments apply for the cherry-pick tests in t35*.sh. Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>