Only add macOS .plist file if contents have changed

[rmartine-ias]

Saving this file triggers a notification saying that a background item was added. This happens on startup, every time. It is annoying.

Tested with:

• Existing file, same as template: no notification
• Existing file, different than template: notification, file added
• No file: notification, file added

Fixes: https://github.com/tmux-plugins/tmux-continuum/issues/118

[dimitur2204]

Would love to see that merged if your fix is working. It is quite annoying

[bruno-]

Thanks!