Example:
I have a repo with a bunch of files in /tmp
in the .gitignore I add /tmp/* and commit and push that
I delete everything in /tmp
arc diff fails with:
STDERR
The following paths are ignored by one of your .gitignore files:
app/tmp/logs
app/tmp/sessions
Use -f if you really want to add them.
fatal: no files added

Not sure if that is the expected behavior, also, there is no way to force it, the -f isn't for arc