branch create: --[no-]commit flag, branchCreate.commit option

[abhinav]

The flag defaults to --commit. Users can use --no-commit on a one-off basis to create a branch without committing changes.

Alternatively, users can set spice.branchCreate.commit to false to make --no-commit the default behavior. They may then use --commit on a one-off basis to commit changes.

Resolves #332

[abhinav]

This change is part of the following stack:

• 347 ◀

_{Change managed by git-spice.}