Unknown branch error when calling GITFNS PRC with single branch

[pamoroso]

Discussed in https://github.com/orgs/Interlisp/discussions/1991

^{Originally posted by pamoroso January 28, 2025}
To learn GITFNS I'm checking out the prc command but I get an unknown branch error.

I initially (FILESLOAD GITFNS). Then, to examine a sample Medley PR such as #1527, I execute prc mth3--Extend-file-COPYRIGHT-for-succesive-owners drafts. This yields the error:

In ERROR:
Unknown branch
mth3--Extend-file-COPYRIGHT-for-succesive-owners drafts

The default project is MEDLEY and the repo's head is master. Do I need to switch to the PR's branch prior to executing prc?