bpo-39546: argparse: Honor allow_abbrev=False for specified prefix_chars (GH-18337)

bpo-39546: argparse: Honor allow_abbrev=False for specified prefix_chars (GH-18337)

When `allow_abbrev` was first added, disabling the abbreviation of
long options broke the grouping of short flags ([bpo-26967](https://bugs.python.org/issue26967)).  As a fix,
b1e4d1b603 (contained in v3.8) ignores `allow_abbrev=False` for a
given argument string if the string does _not_ start with "--"
(i.e. it doesn't look like a long option).

This fix, however, doesn't take into account that long options can
start with alternative characters specified via `prefix_chars`,
introducing a regression: `allow_abbrev=False` has no effect on long
options that start with an alternative prefix character.

The most minimal fix would be to replace the "starts with --" check
with a "starts with two prefix_chars characters".  But
`_get_option_tuples` already distinguishes between long and short
options, so let's instead piggyback off of that check by moving the
`allow_abbrev` condition into `_get_option_tuples`.

https://bugs.python.org/issue39546