Ignore unmodified files when using `--new-pr` or `--update_pr`

[Flamefire]

When including an unmodified EasyConfig with --new-pr an error is shown that a commit message is required because this EC is modified which is not the case.

Adjust the copy_* functions to ignore any unmodified file. This especially omits them in the file_info struct returned that is then used to determine commit message/title etc.

Fixes #4751

Note: While working on this I noticed that new EasyConfigs that are dependencies of added EasyConfigs are ignored in the commit message. I propose to populate file_info instead of the dep_info at https://github.com/easybuilders/easybuild-framework/blob/b37f70756960c1686875974f8381491df280cc00/easybuild/tools/github.py#L1071

This would also fix the possibly incomplete debug log message Staging all <n> new/modified easyconfigs that doesn't account for those ECs.