getGitReposInFolder: fix async folder filtering

[lbogdan]

Folder filtering in Git.getGitReposInFolder()

const folders = await Promise.all<string>(filteredItems.filter(async item => (await fs.stat(item)).isDirectory()));

is not working as intended, because the filter() callback returns a Promise, which is a truthy value, so filter() just passes filteredItems through, and Promise.all() does the same (as the array values are strings, not Promises), so in the end folders is a copy of filteredItems.

Fixes #143 .

Note: I adapted asyncFilter() implementation from this StackOverflow answer.