BUG: Subtle bug in merge_sort with length 1 lists (note complication)

[seberg]

When sorting a length 1 list, the returned list is identical to the input list. This leads to a subtle inconsistency and a copy should be returned.

As a toy example, if the code for example, replaces the largest element later:

data = [1, 2]  # or [1]

sorted_data = merge_sort(data)
sorted_data[-1] = 999

then the original data is normally not be modified. But if the input has length 1, it will be.

Minimal reproducer

from listwiz.sorting import merge_sort 

in1 = [1]

res1 = merge_sort(in1)

assert res1 is not in1  # assertion should not fail

Working on the issue

When fixing this issue, make sure to add a new test function (after test_mergesort_empty!).

NOTE: If you work on this issue, changes are likely to conflict with the issue gh-8. This means that whichever PR is merged second, will need to deal with the "merge conflict".
Because of that and we may wait with merging this until the other PR is merged.