Feature request: Configurable prefixes for git commit --fixup and --squash

Hi, I'm not sure if this was already requested somewhere (a quick - but admittedly not thorough - search did not reveal anything in that direction), but I really miss an option to configure the prefixes generated by `git commit (--fixup | --squash) <commit>` and picked up by `git rebase -i --autosquash`.

My reasoning is that in our project we use GitHub + Travis to test-build our pull requests, but we don't want to spam the CI server with builds that are just fixups to previous changes (which are uploaded so reviewers can track the changes to the original PR). Now, Travis has the option to not build a commit if there is the string `[ci skip]` in the commit message (sadly also not configurable) so it would be really great for my workflow if I could just add this string to the message generated by `--fixup`.

Thanks in advance for your consideration.

Kind Regards, Martine Lenders