Fix realpath() failing on Windows.

[AnrDaemon]

In certain scenarios, realpath() failing to completely unwind all symlinks, resulting in Composer producing autogenerated mappings similar to this:

config(vendor-dir):C:\Users\anrdaemon\Documents\.github\1/vendor
$vendorPath:C:/home/Daemon/Documents/.github/1/vendor
$targetDir:C:/home/Daemon/Documents/.github/1/vendor/composer
realpath($targetDir):C:\Users\Daemon\Documents\.github\1\vendor\composer
$vendorPathCode:dirname(dirname(dirname(dirname(dirname(dirname(dirname(__DIR__))))))).'/home/Daemon/Documents/.github/1/vendor'

After a rather silly looking patch, it works much better:

config(vendor-dir):C:\Users\anrdaemon\Documents\.github\1/vendor
$vendorPath:C:/Users/Daemon/Documents/.github/1/vendor
$targetDir:C:/Users/Daemon/Documents/.github/1/vendor/composer
realpath($targetDir):C:\Users\Daemon\Documents\.github\1\vendor\composer
$vendorPathCode:dirname(__DIR__)

[Seldaek]

Thanks for the patch, but I do wonder.. is there a case where two realpath() is not enough and we need 3? or 4? Also is there a way to write a unit test for this? It'd be quite nice to avoid future regressions.

[staabm]

on which phpversion does this issue occur?

[AnrDaemon]

As this is a bug in PHP itself, it MAY be possible.
Speaking of bugs,

1. https://bugs.php.net/bug.php?id=72642
2. https://bugs.php.net/bug.php?id=72738

Pick your one.

[AnrDaemon]

@staabm , on all windows versions.

[AnrDaemon]

My hope with this patch (yes, I've considered doing a heavier version of it accounting for possible multiple unresolved paths) is that there won't be many people overlinking their filesystems, as conditions for this issue to arise are rather specific.

[SoboLAN]

@AnrDaemon Maybe reference the link to this ticket in the code comment ?

[AnrDaemon]

@SoboLAN , good idea, done.
@Seldaek , I'm not sure about unit tests. Perhaps, someone could scavenge the needed information from the reproduction code provided in https://bugs.php.net/bug.php?id=72738 ?

[Seldaek]

Oh sorry it seems I forgot to close this when I merged your commit. I merged as c774d41 and added the ref in 902a5c3