This issue is reproducible with both gitg-3.20.1-1.fc24.x86_64 and the latest git master (as of today).

Steps to reproduce:

1. Launch a terminal
2. Create a git repo: mkdir aaa; cd aaa; git init
3. Launch gitg
4. Add the just created repo to gitg
5. Switch back to the terminal: echo "*.pdf -diff" > .gitattributes
6. Switch back to gitg
7. Go to the commit view of the new repo
8. Double-click the .gitattributes file to stage it
9. Click the commit button

Expected results:

- The .gitattributes file is staged
- gitg allows to commit the .gitattributes file

Actual results:

- The .gitattributes file is not staged according to 'git status'
- gitg says there are no changes to commit