get_dirs_from_args() breaks on Windows with options that include full paths

[nicoddemus]

pytest-xdist rsync feature uses options in the command line that looks like files, but can themselves point to full paths:

C:\\Users\\appveyor\\AppData\\Local\\Temp\\1\\pytest-of-appveyor\\pytest-0\\test_popen_rsync_subdir0\\popen\\chdir=C:\\Users\\appveyor\\AppData\\Local\\Temp\\1\\pytest-of-appveyor\\pytest-0\\test_popen_rsync_subdir1\\dest

(From https://ci.appveyor.com/project/pytestbot/pytest-xdist/build/job/r7el1489cu4ipfmp).

Preparing a PR shortly. 👍