gh-117639: Special case for "" & "." in posixpath.abspath()

[nineteendo]

Benchmark:

# test.sh
echo "abspath()" && python -m timeit -s "import before.posixpath" "before.posixpath.abspath('foo')" && python -m timeit -s "import after.posixpath" "after.posixpath.abspath('foo')"
echo "abspath('.')" && python -m timeit -s "import before.posixpath" "before.posixpath.abspath('.')" && python -m timeit -s "import after.posixpath" "after.posixpath.abspath('.')"
echo "relpath()" && python -m timeit -s "import before.posixpath" "before.posixpath.relpath('foo', 'bar')" && python -m timeit -s "import after.posixpath" "after.posixpath.relpath('foo', 'bar')"
echo "relpath('foo')" && python -m timeit -s "import before.posixpath" "before.posixpath.relpath('foo')" && python -m timeit -s "import after.posixpath" "after.posixpath.relpath('foo')"
echo "relpath('/')" && python -m timeit -s "import before.posixpath" "before.posixpath.relpath('/')" && python -m timeit -s "import after.posixpath" "after.posixpath.relpath('/')"

abspath()
20000 loops, best of 5: 17.7 usec per loop # before
20000 loops, best of 5: 17.4 usec per loop # after
# -> 1.02x faster
abspath('.')
20000 loops, best of 5: 18.1 usec per loop # before
20000 loops, best of 5: 15.7 usec per loop # after
# -> 1.15x faster (for '' & '.')
relpath()
10000 loops, best of 5: 39.4 usec per loop # before
10000 loops, best of 5: 39.8 usec per loop # after
# -> no difference
relpath('foo')
5000 loops, best of 5: 38.9 usec per loop # before
10000 loops, best of 5: 37.4 usec per loop # after
# -> 1.04x faster (for a single relative path)
relpath('/')
10000 loops, best of 5: 22 usec per loop # before
10000 loops, best of 5: 20.7 usec per loop # after
# -> 1.06x faster (for a single absolute path)

• Issue: Special case for "" & "." in posixpath.abspath() #117639