Shell search provider doesn't find files outside of home directory

Copied from Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=760366

Steps to reproduce

1. In a terminal, run sudo mkdir /home/spare && sudo chown $USER:$USER /home/spare
2. Place a document file (like a pdf or a Word/OpenOffice doc) in /home/spare and ~/Documents.
3. Enter Settings->Search.
4. Enable "Documents" and "Files" as search providers.
5. Click the gear button in the bottom-right corner, and enable "Documents" as a search path in the "Places" tab, and add /home/spare as a search path in the "Other" tab.
6. Wait a minute to let Tracker index everything. Run tracker daemon to check whether it's finished or not.
7. In the Overview, search for the file you added to /home/spare and ~/Documents.

Reproducible in:

• Flatpak unstable: yes
• Other: nautilus-3.28.1-1.fc28.x86_64

Current behavior

The file will only appear as a Documents search result, i.e. Documents' search provider does not require files to be under /home/$USER, but Nautilus' does.

Expected behavior

The file will appear in the Overview as a Nautilus search result, and also as a Documents search result.

Additional information

This is not a Tracker bug, as Tracker is able to successfully index files outside of /home/$USER. For example, after following the reproduction steps, tracker info /home/spare/<file> returns metadata of that file, proving that it was indexed.

A useful comment from the original bug report is #8 from António Fernandes:

Looking at the code in nautlus-shell-search-provider.c, it seems that the user's home is indeed the scope of the search:

home = g_file_new_for_path (g_get_home_dir ());
...
nautilus_query_set_location (query, home);