Reverse / fix meaning of "+/-" in error diffs

The convention is "assert result is expected".  Pytest's error diffs now
reflect this. "-" means that sth. expected is missing in the result and
"+" means that there are unexpected extras in the result.

Fixes: #3333

doc/en/example/reportingdemo.rst

@@ -81,8 +81,8 @@ Here is a nice run of several failures and how ``pytest`` presents things:
         def test_eq_text(self):
     >       assert "spam" == "eggs"
     E       AssertionError: assert 'spam' == 'eggs'
-    E         - spam
-    E         + eggs
+    E         - eggs
+    E         + spam
 
     failure_demo.py:45: AssertionError
     _____________ TestSpecialisedExplanations.test_eq_similar_text _____________
@@ -92,9 +92,9 @@ Here is a nice run of several failures and how ``pytest`` presents things:
         def test_eq_similar_text(self):
     >       assert "foo 1 bar" == "foo 2 bar"
     E       AssertionError: assert 'foo 1 bar' == 'foo 2 bar'
-    E         - foo 1 bar
+    E         - foo 2 bar
     E         ?     ^
-    E         + foo 2 bar
+    E         + foo 1 bar
     E         ?     ^
 
     failure_demo.py:48: AssertionError
@@ -106,8 +106,8 @@ Here is a nice run of several failures and how ``pytest`` presents things:
     >       assert "foo\nspam\nbar" == "foo\neggs\nbar"
     E       AssertionError: assert 'foo\nspam\nbar' == 'foo\neggs\nbar'
     E           foo
-    E         - spam
-    E         + eggs
+    E         - eggs
+    E         + spam
     E           bar
 
     failure_demo.py:51: AssertionError
@@ -122,9 +122,9 @@ Here is a nice run of several failures and how ``pytest`` presents things:
     E       AssertionError: assert '111111111111...2222222222222' == '111111111111...2222222222222'
     E         Skipping 90 identical leading characters in diff, use -v to show
     E         Skipping 91 identical trailing characters in diff, use -v to show
-    E         - 1111111111a222222222
+    E         - 1111111111b222222222
     E         ?           ^
-    E         + 1111111111b222222222
+    E         + 1111111111a222222222
     E         ?           ^
 
     failure_demo.py:56: AssertionError

src/_pytest/assertion/__init__.py

@@ -168,4 +168,4 @@ def pytest_sessionfinish(session):
 
 
 def pytest_assertrepr_compare(config, op, left, right):
-    return util.assertrepr_compare(config=config, op=op, left=left, right=right)
+    return util.assertrepr_compare(config=config, op=op, result=left, expected=right)