CONTRIBUTING.md: simplify rebasing instructions
CONTRIBUTING.md: simplify rebasing instructions

Makes the instructions easier to remember (and type) using the
`git rebase --onto A...B` syntax to find the merge base between A and B
(which has been in git for at least 10 years).

We also assume that the merge base between staging and master is also
the merge base between staging and the current branch (since it is based
on master), and giving master as the <upstream> branch makes git
consider the commits in the current branch that are not in master, so
there's no need to compute the merge base between master and the current
branch.

In the same spirit of discouraging copy-and-paste, use a placeholder
name for the current branch instead of `$(git branch --show-current)`.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs