path.relative method returns wrong path when UNC path is provided

[amatas]

It seems that versions prior v6 are not affected.

• Version: v6
• Platform: windows
• Subsystem: path

Tests:

• Windows 10 with node 4.5.0:

  c:\>node -e "console.log(require('path').relative('.','\\\\VBOXSRV\\vagrant'))"

returns: \\VBOXSRV\vagrant\

• Windows 10 with node 6.5.0:

  c:\>node -e "console.log(require('path').relative('.','\\\\VBOXSRV\\vagrant'))"

returns: VBOXSRV\vagrant\(the leading backslashes are missing)
expected: \\VBOXSRV\vagrant\

Related issue: nodejs/nan#601