Pipeline creation should specify branch name when initialising git repository

[alexlyttle]

Description of feature

When creating a new pipeline with nf-core pipelines create, the new repository is initialised without specifying initial branch name. A small improvement would be to specify the initial branch name with the GitPython API. Currently, the initial branch should be master (and then main in the future as planned). Depending on a user's Git settings, the initial branch could be something other than this.

This change should be as simple as modifying line 440 of create.py to include the initial_branch keyword argument.

Some context: I am trying to find "gotchas" that might cause confusion as I am currently helping researchers build their own pipelines.