Update linter command to support diff linting

[berndverst]

Description

Now instead of make lint which will complain about historical lint errors, you can also run:

get fetch upstream # update your upstream references
LINT_BASE=upstream/main make lint

This will only lint the commits you added since the commit pointed to by LINT_BASE. Under the hood this uses git rev-parse so you can use friendly names like tags, branch names or simply commit IDs.