fix issue364: shorten and enhance tracebacks representation by default.

The new "--tb=auto" option (default) will only display long tracebacks
for the first and last entry.  You can get the old behaviour of printing
all entries as long entries with "--tb=long".  Also short entries by
default are now printed very similarly to "--tb=native" ones.

testing/python/collect.py

@@ -641,7 +641,7 @@ class TestTracebackCutting:
         assert "x = 1" not in out
         assert "x = 2" not in out
         result.stdout.fnmatch_lines([
-            ">*asd*",
+            " *asd*",
             "E*NameError*",
         ])
         result = testdir.runpytest("--fulltrace")