UNC paths are not handled appropriately by `path.relative`

[amanda-mitchell]

• Version: 6.3.1
• Platform: 64 bit Windows 10
• Subsystem: path.js

When calling path.relative, if the to parameter refers to a UNC (network) path, the result is incorrect.

Steps to Reproduce

Run the following program:

var path = require('path');

console.log(path.relative('c:\\some\\directory', '\\\\localhost\\c$\\another\\directory'));

Expected Result

The console output should be \\localhost\c$\another\directory because no relative path exists between the two paths.

Actual Result

The console output is localhost\c$\another\directory: the leading double backslash is omitted.

Extra Notes

The output is correct for two UNC paths to the same host. The output is incorrect for two UNC paths to different hosts.

[gibfahn]

cc/ @nodejs/platform-windows

[joaocgreis]

This was fixed in v6.8.0 (duplicate of #8444, fixed in PR #8523).

I'll close the issue, but we can reopen if there's something else to discuss.