Merge branch 'jk/set-upstream-error-cases'

The handing by "git branch --set-upstream-to" against various forms of errorneous inputs were suboptimal. * jk/set-upstream-error-cases: branch: give advice when tracking start-point is missing branch: mention start_name in set-upstream error messages branch: improve error message for missing --set-upstream-to ref branch: factor out "upstream is not a branch" error messages t3200: test --set-upstream-to with bogus refs