Disable push branches drop-down in Git tab if modifying of Git/Mercurial config files is disabled in the General tab

Reproduction steps:

1. Go to SourceTree Preferences and click Git tab
2. Change Push branches from matching to current
3. Close preferences
4. Click push icon
5. Current behavior: All checked-out branches are selected
6. Expected behavior: Just the current branch shall be selected for push
7. Also, if you reopen preferences, the Push branches setting is set to matching instead of current