buildbot-effects: bug in --branch

When running an effect using buildbot-effects on a given branch `my-great-branch` should be the same as running `--rev` on the newest commit on the branch. For example ```console buildbot-effects --branch <my-great-branch> run <my-effect> ``` ```console buildbot-effects --rev <my-sha> run <my-effect> ``` I tried this on a PR and got two different SHA values ```console {branch:test/hil-retry,ref:null,rev:8ea194a11f3004fd7c3951af03935000691d4f8b,tag:null} ``` versus ```console {branch:main,ref:null,rev:ed72ed207cb3cd1ef0bee2a1de71b7a7e43ae7d5,tag:null} ``` I think there may be a bug in the way git resolves SHA.