bpo-26967: fix flag grouping with allow_abbrev=False (GH-14316)

The `allow_abbrev` option for ArgumentParser is documented and intended to disable support for unique prefixes of --options, which may sometimes be ambiguous due to deferred parsing.

However, the initial implementation also broke parsing of grouped short flags, such as `-ab` meaning `-a -b` (or `-a=b`).  Checking the argument for a leading `--` before rejecting it fixes this.

This was prompted by pytest-dev/pytest#5469, so a backport to at least 3.8 would be great 😄  
And this is my first PR to CPython, so please let me know if I've missed anything!


https://bugs.python.org/issue26967

Lib/test/test_argparse.py

@@ -785,6 +785,25 @@ class TestOptionalsDisallowLongAbbreviation(ParserTestCase):
         ('--foonly 7 --foodle --foo 2', NS(foo='2', foodle=True, foonly='7')),
     ]
 
+
+class TestDisallowLongAbbreviationAllowsShortGrouping(ParserTestCase):
+    """Do not allow abbreviations of long options at all"""
+
+    parser_signature = Sig(allow_abbrev=False)
+    argument_signatures = [
+        Sig('-r'),
+        Sig('-c', action='count'),
+    ]
+    failures = ['-r', '-c -r']
+    successes = [
+        ('', NS(r=None, c=None)),
+        ('-ra', NS(r='a', c=None)),
+        ('-rcc', NS(r='cc', c=None)),
+        ('-cc', NS(r=None, c=2)),
+        ('-cc -ra', NS(r='a', c=2)),
+        ('-ccrcc', NS(r='cc', c=2)),
+    ]
+
 # ================
 # Positional tests
 # ================