Tidy formatting of assertion truncation

Part two of pytest-dev#1512. Update the format
of the truncation message to help make it clear that pytest truncates the
entire assertion output when verbosity < 2.

Author: mattduck

_pytest/assertion/__init__.py

@@ -131,14 +131,21 @@ def callbinrepr(op, left, right):
             config=item.config, op=op, left=left, right=right)
         for new_expl in hook_result:
             if new_expl:
+
+                # Truncate lines if required
                 if (sum(len(p) for p in new_expl[1:]) > 80*8 and
                         item.config.option.verbose < 2 and
                         not _running_on_ci()):
                     show_max = 10
-                    truncated_lines = len(new_expl) - show_max
-                    new_expl[show_max:] = [py.builtin._totext(
-                        'Detailed information truncated (%d more lines)'
-                        ', use "-vv" to show' % truncated_lines)]
+                    truncated_count = len(new_expl) - show_max
+                    new_expl[show_max - 1] += " ..."
+                    new_expl[show_max:] = [
+                        py.builtin._totext(""),
+                        py.builtin._totext('...Full output truncated (%d more lines)'
+                                           ', use "-vv" to show' % truncated_count
+                        ),
+                    ]
+
                 new_expl = [line.replace("\n", "\\n") for line in new_expl]
                 res = py.builtin._totext("\n~").join(new_expl)
                 if item.config.getvalue("assertmode") == "rewrite":