fix: prevent "too many open files" errors from git ignored check

[cseickel]

fixes #1125, fixes #997

The problem was actually cuased by calling out to git too many times when checking the git ignored status of files. This only happens when there are many folders being shown at once. The solution was to limit the number of concurrent running jobs to 50.

I also snuck in a config option to completely disable checking gitignore when searching, as this has a big performance impact on the "search as you type" mode. Personally, I'd rather it be fast than be able to mark gitignored items.

require("neo-tree").setup({
  filesystem = {
    check_gitignore_in_search = false, -- Check gitignore status for files/directories when searching.
                                       -- Setting this to false will speed up searches, but gitignored
                                       -- items won't be marked if they are visible.
  }
})

The default value is true so as not to change the existing behavior.