Look through and resolve todos

[a-roberts]

There are plenty of todos in the codebase so far, it'll be useful to look through these and see if there's any low-hanging fruit, or anything that's quite bad.

This would be an excellent first issue for somebody looking to learn our dev process and to get things fixed 😄

[JustinKuli]

List of TODOs:

• pkg/cmd/promote.go:154: // TODO: make this a multierror with both errors?
• pkg/avancement/pull_request.go:10:// TODO: OptionFuncs for Base and Title?
• pkg/avancement/pull_request.go:11:// TODO: For the Head, should this try and determine whether or not this is a
• pkg/avancement/service_manager.go:84:// TODO: make this a command-line parameter defaulting to "master"
• pkg/avancement/service_manager.go:249: // TODO: improve this error message
• pkg/git/copy_file_test.go:13:// TODO: fileCopy does a number of things, and catches a lot of error cases.
• pkg/git/repository.go:209: // TODO: more sophisticated logging.
• pkg/git/repository.go:216:// TODO: this probably needs specialisation for GitLab URLs.
• pkg/git/repository.go:217:// TODO: should we process "git@github.com" urls?, this would require SSH keys.

I couldn't find open issues for any of them. I haven't looked at all of them, but I'm currently working on pkg/avancement/service_manager.go:249, which is definitely the smallest of the bunch.

It seems like pkg/avancement/service_manager.go:84 and part of pkg/avancement/pull_request.go:10 would make sense if we added from-branch and to-branch options? (Created #100 for this)