Mantle outputs not working with .yaml file extension

[christopher-buss]

Describe the bug
Mantle outputs does not work with .yaml.

If I run mantle outputs mantle.yaml I get a:
Unable to determine git branch. Are you in a git repository?
The filename, directory name, or volume label syntax is incorrect. (os error 123)

[blake-mealey]

By default, Mantle attempts to determine the environment based on the Git branch. It looks like you're not running from a Git context, so you'll need to explicitly pass the environment in with --environment <name> (or -e <name>) for short.

[christopher-buss]

If I rename the file to mantle.yml and run the command, it works without an issue, without needing to specific the environment.