Testing against non-tracked branches fails

[wlynch]

[/tmp/checkout ‹main›]$ git branch --contains master 
error: malformed object name master
[/tmp/checkout ‹main›]$ git co master
branch 'master' set up to track 'origin/master'.
Switched to a new branch 'master'
[/tmp/checkout ‹master›]$ git co main
Switched to branch 'main'
Your branch is up to date with 'origin/main'.
[/tmp/checkout ‹main›]$ git branch --contains master
* main
  master