This adds returning the commit id from read_commit, and a new function
read_recipe_and_id() that returns the commit id and the recipe in a
tuple.
If the commit is passed in, it is used as is. If no commit is passed in
it finds the most recent commit for the file on the selected branch and
returns that.
Missing recipes now raise a RecipeError with an informative message.
eg. "No commits for missing-recipe.toml on the master branch."
Recipe should have its version bumped based on the version from the
previous commit, and not be bumped on the first commit. Fix the code and
the tests.
It appears that with libgit2 v0.24.6 reverse causes it to list them
newest first. In 0.25.1 it lists them oldest first. On both versions
just using SortMode.TIME gives the desired result of oldest first.
