Use original commit messages for patch commits if possible

[TomasTomecek]

We've got a report from @karelzak via email that he'd love if downstream patches contained actual commit messages where possible.

The problem is that rpm "ignores" real commit messages when applying them via %autosetup -Sgit. We already have code in packit source-git init which post-processes the patch commits:

https://github.com/packit/packit/blob/2a9af757df7b0ad47016bdb8fe86a7924e98595e/packit/source_git.py#L196-L216

So we probably just need to "backport" a similar functionality here and implement this in packit init as well.