gh-117338: Handle leading // for posixpath.realpath

[nineteendo]

Benchmark:

python -m timeit -s "from test import realpath1" "realpath1('foo/../../..')" && python -m timeit -s "from test import realpath2" "realpath2('foo/../../..')"
10000 loops, best of 5: 23.5 usec per loop # before
10000 loops, best of 5: 23.6 usec per loop # after
# -> no difference

• Issue: Handle leading // for posixpath.realpath #117338