Add option to git pull --rebase before scan/push of pkgdev push

[ajakk]

Given the pace of commits to gentoo.git, it's pretty common to need to git pull --rebase before being able to successfully push something. Thus, it would be great for pkgdev push to be able to do this for us automatically.

[arthurzam]

OK, that's sound good. I think naming it --pull without short options will be good.

@ajakk do you think the git pull --rebase should be done before pkgcheck scan or after? On one hand, it is logical to perform it before scanning, so we check for issues after rebasing. But on the other hand, scan might take long time, in which another dev might push.

[ajakk]

There's a race condition either way between getting to the local push and other developer's pushes. I don't think it matters much in practice, but I'd prefer the scan after the pull to catch any new changes from the remote.

Also, if it's not too much trouble, can there be a config option for this? I wouldn't mind this doing a git pull --rebase unconditionally whenever I push.

[arthurzam]

@ajakk thanks for the answer, opened a PR with this implemented (with instructions how to add to config). I will wait for sam (or you if you agree) to review the strings used.