bpo-26967: fix flag grouping with allow_abbrev=False (GH-14316) (GH-14759)

bpo-26967: fix flag grouping with allow_abbrev=False (GH-14316) (GH-14759)

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/pytestGH-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
(cherry picked from commit dffca9e925ee5c3072663cbe8d4d4768406d5307)

Co-authored-by: Zac Hatfield-Dodds <Zac-HD@users.noreply.github.com>