Fix using gitignore and absolute options at the same time on Windows

[rwjblue]

When globby is invoked with globby('*', { absolute: true, gitignore: true }) on Windows systems, the assertion added in 4af038e always triggers. This is because when we default the cwd in normalizeOptions, we didn't convert from C:\\ style paths to forward slashes.

A simple work around for users hitting this issue, is to provide a cwd option that has already been converted:

globby('*', { absolute: true, gitignore: true, cwd: slash(process.cwd()) })

---

The test added here was failing on Windows, example output prior to the fixes:

  test » gitignore option with absolute option
  C:\Users\travis\build\rwjblue\globby\gitignore.js:52
   51:
   52:     throw new Error(`Path ${p} is not in cwd ${cwd}`);
   53:   }
  Rejected promise returned by test. Reason:
  Error {
    message: 'Path C:/Users/travis/build/rwjblue/globby/a.tmp is not in cwd C:\\Users\\travis\\build\\rwjblue\\globby',
  }

Related to #133 (though it isn't clear that this fixes all of the scenarios that #133 is exploring).

[jamiekyle-eb]

I found a library path-is-inside that is supposed to do a better job than the filePath.startsWith(cwd) check I wrote on https://github.com/sindresorhus/globby/blob/master/gitignore.js#L48

[rwjblue]

Ya, looks like that one deals with case sensitivity by platform and normalizing trailing slashes, but still effectively is doing .startsWith (it uses .lastIndexOf(needle, 0) === 0). I think it would fail in the same way described in my description above (because it would not convert the backslashes in process.cwd() to forward slashes which is what fast-glob is giving us).

I'm happy to pull in that package and use it here, I just think we'll still need to normalize the process.cwd() before comparison.

[sindresorhus]

I have a similar package we could use: https://github.com/sindresorhus/is-path-inside It fixes a couple of shortcomings of path-is-inside.

Moving this to a new issue: #140