[performance] Look into reducing number of API calls during create

[shevron]

When calling create we are doing quite a lot of API calls (see analysis in #17). Look into ways of reducing the number of calls.

• Perhaps, using GraphQL mutations (if supported by GitHub) can work to create a tree of objects in a single call.
• See if we can avoid calling create_file (not likely) to create the base commit for the repo
• See if we can avoid calling get_branch to get the sha of the tip of the main branch.

[shevron]

This applies to update as well as to create.