fix(pack): higher priority to .npmignore over .gitignore on windows

[pieh]

Summary

This fixes yarn pack not ignoring .gitignore when .npmignore is avaialable on Windows. Initial fix was done in #3538, but it used string concatenation to join paths which make it unfortunately fail for Windows which uses different path separator than other systems. It was reported as not fixed in #685 (comment) (OS: Windows) in initial issue after issue was closed as fixed.

Test plan

First I tested if tarball created by yarn pack is correct (so it ignores .gitignore when I have .npmignore).

To demonstrate here, I added extra console output here https://github.com/yarnpkg/yarn/blob/master/src/cli/commands/pack.js#L89 logging dotIgnoreFiles and here's before / after:

Before:

D:\dev\yarn-test>yarn pack
yarn pack v1.7.0
dotIgnoreFiles [ { relative: '.gitignore',
    basename: '.gitignore',
    absolute: 'D:\\dev\\yarn-test\\.gitignore',
    mtime: 1528836250698 },
  { relative: '.npmignore',
    basename: '.npmignore',
    absolute: 'D:\\dev\\yarn-test\\.npmignore',
    mtime: 1528836254147 } ]
success Wrote tarball to "D:\\dev\\yarn-test\\yarn-test-v1.0.0.tgz".
Done in 0.45s.

After:

D:\dev\yarn-test>yarn pack
yarn pack v1.7.0
dotIgnoreFiles [ { relative: '.npmignore',
    basename: '.npmignore',
    absolute: 'D:\\dev\\yarn-test\\.npmignore',
    mtime: 1528836254147 } ]
success Wrote tarball to "D:\\dev\\yarn-test\\yarn-test-v1.0.0.tgz".
Done in 0.06s.

.gitignore is correctly omitted if .npmignore is present

[hulkish]

LGTM

[arcanis]

Thanks! The change looks safe so I'm going to merge it, but it would be great if you could add a test in __tests__, @pieh

[pieh]

Let me know if those tests are still needed after #6149. I had started on something like pieh@a269136 but didn't like changing filterOverridenGitignores function signature for passing path.posix.join or path.win32.join just for tests

[arcanis]

Yep, it would be nice to have a new test that would ensure that the .npmignore always has precedence over .gitignore