Unequal location hints point at equal sections

I've encountered a situation where pytest incorrectly reports the character positions of mismatched characters in two strings being compared. Consider this test:

import textwrap

expected = textwrap.dedent("""
    --w-r---wx 1 0     0     999999 2019-02-01 bar
    drwxr-xr-x 2 0     0         64 2019-02-01 biz
    -rw-r--r-- 1 0     0        100 2019-02-01 foo
    """).lstrip().format(**locals())

actual = textwrap.dedent("""
    --w-r---wx 1 0     0     999999 2019-02-01 bar
    drwxrwxr-x 2 0     0         64 2019-02-01 biz
    -rw-rw-r-- 1 0     0        100 2019-02-01 foo
    """).lstrip().format(**locals())


def test_equal():
    assert expected == actual

Running pytest on that file emits:

draft $ pip-run -q pytest -- -m pytest -vv test-cmp.py                                                                                                                                   
================================================================================== test session starts ==================================================================================
platform darwin -- Python 3.7.2, pytest-4.2.0, py-1.7.0, pluggy-0.8.1 -- /Library/Frameworks/Python.framework/Versions/3.7/bin/python3
cachedir: .pytest_cache
rootdir: /Users/jaraco/draft, inifile:
plugins: xonsh-0.8.9
collected 1 item                                                                                                                                                                        

test-cmp.py::test_equal FAILED                                                                                                                                                    [100%]

======================================================================================= FAILURES ========================================================================================
______________________________________________________________________________________ test_equal _______________________________________________________________________________________

    def test_equal():
>       assert expected == actual
E       AssertionError: assert '--w-r---wx 1...9-02-01 foo\n' == '--w-r---wx 1 ...9-02-01 foo\n'
E           --w-r---wx 1 0     0     999999 2019-02-01 bar
E         - drwxr-xr-x 2 0     0         64 2019-02-01 biz
E         ?        ---
E         + drwxrwxr-x 2 0     0         64 2019-02-01 biz
E         ?  +++
E         - -rw-r--r-- 1 0     0        100 2019-02-01 foo
E         ?        ---
E         + -rw-rw-r-- 1 0     0        100 2019-02-01 foo
E         ? +++

test-cmp.py:17: AssertionError
=============================================================================== 1 failed in 0.06 seconds ================================================================================

The failure is expected, but the reporting that it emits is not. As you can see, the ^^^ hints are pointing to sections that are the same in the actual and expected results.

As I'd never encountered this before, I originally thought it was a unittest bug, but this example doesn't use unittest at all.

Start by running the reproducer in test-cmp.py with pytest and compare the displayed hints with the differing characters in the two multiline strings. Trace pytest's assertion-reporting path for string comparisons. Done means the location hints identify the mismatched sections rather than equal sections, with the reproducer still passing as an appropriate regression test.