[tool] easy branch rebase

For devs who have branches to rebase every so often (e.g. when your PR branch is getting out of sync with master), I made a little helper tool: https://github.com/stas00/git-tools/tree/master/git-rebase

It does a quick rebase/merge w/ sanity checks/etc. Saves a lot of time for me - hope you find it useful too.