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 :smile:  
And this is my first PR to CPython, so please let me know if I've missed anything!


https://bugs.python.org/issue26967