[barbuk.theme] Allow setting git current user and add a default icon

[BarbUk]

Description

Allow setting git current user and add a default icon for the theme barbuk

Motivation and Context

SCM_GIT_SHOW_CURRENT_USER was set to false in the theme.
Configuring SCM_GIT_SHOW_CURRENT_USER to true had no effect.

The default is false, so pulling this update will not change behavior for user using this theme.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[seefood]

Seems legit...