Whippet gets confused if both `main` and `master` exist

[RobjS]

To recreate:

• Create a new plugin repo, with both main and master branches. Make master the default
• Push an older version of the plugin to main, and a newer version to master (i.e. so that the older version does not exist on the commit history of main)
• Point your whippet.json file at that repo (with no specific reference beyond the repo source)
• Run whippet deps update. Whippet will take the commit hash reference added to whippet.lock from main, but try and pull the plugin from master, using that reference. This will result in a fatal: reference is not a tree: [commit-hash-from-master] error when running whippet deps install or whippet deps update

It appears that, if main exists, Whippet assumes it's the default branch, but that may not actually be the case. It should probably just use the default branch at all times, and allow that default branch to be either main or master.

[snim2]

@RobjS where we've had this problem, we haven't always been consistent about whether main or master is the default branch. If a plugin repo has both, I'm wondering whether Whippet should just exist with an error message?