don't allow using --pr-commit-msg when only adding new files with --new-pr (unless --force is used)

[branfosj]

We should force the generated message in these cases.

An alternative to raising an error (as implemented here) would be to generate the default PR title and replace the commit_msg in these cases while printing a warning.

[smoors]

should we also force re-generating the PR title if easyconfigs are renamed or deleted? because in that case the title is not correct anymore

[Micket]

@smoors We'd have to check that it wasn't modifying any file before doing that. But why not

[boegel]

I'm not sure if this is a good idea...

I think it's good to have the option to use --pr-commit-msg to provide a more meaningful commit message if you feel the auto-generated doesn't make sense, or can be improved.

Maybe we can require --force to not completely disallow but, only discourage it?

[branfosj]

I'm not sure if this is a good idea...

I think it's good to have the option to use --pr-commit-msg to provide a more meaningful commit message if you feel the auto-generated doesn't make sense, or can be improved.

Maybe we can require --force to not completely disallow but, only discourage it?

61aff50 allows bypassing the error if --force is used.

I explicitly want this to be an error as I want people to see it and act on it. I'm deliberately not saying that you can use --force to bypass the check for the same reason.

[smoors]

works as advertised, lgtm