Use realpath() to find the parent of a relative path

[pjcdawkins]

I noticed Drush 9 would find my Drupal site with any of these:

$ cd site
$ drush -r ../site/web status
$ drush -r /Users/patrick/site/web status
$ drush -r web/../web status

but curiously not with:

$ cd site
$ drush -r web status

My composer.json lives in site, while autoload.php and core live under web, based on https://github.com/platformsh/template-drupal8

I noticed the shiftPathUp() method does not account for relative paths.

This PR is a small modification to shiftPathUp() which means it can use the realpath() if necessary to normalize a relative path.

If incorporated into Drush 9, this would allow a Platform.sh CLI issue to be resolved quite nicely platformsh/legacy-cli#772

[pjcdawkins]

Rebasing/updating to add a failing test, and then the fix

[weitzman]

FWIW I’ve had problems in the past when using real path on windows. Not sure if that applies here. Would be ideal to find a different approach.

[pjcdawkins]

If realpath() here fails by returning false or the original path, then there is no change in behavior.

Windows only needs to make the path real "enough" to find the parent directory each time, if any, and this method is called until there is no parent directory, so the bug mentioned here does not apply.

realpath() only works on existing files - fortunately in this case existing files are all we care about.

realpath() is used frequently elsewhere in this class, to resolve symlinks.

[pjcdawkins]

Changes approved. I'd still find this really useful. @webflo would you mind reviewing?

[weitzman]

Any thoughts @webflo?