Fix check_comfy_repo

[huchenlei]

Currently check_comfy_repo is returning Tuple[bool, Optional[git.Repo]], but most call site is treating the return value as a bool, so the check at most callsites just always pass, because the a tuple of 2 items always evals to True.

This PR does following changes:

• Rename check_comfy_repo to get_comfy_repo as the git.Repo is the most important return value
• Simplify the return value to Optional[git.Repo] as the bool can be obtained from nullity of Optional[git.Repo]
• Change call site where a tuple of 2 itmes are expected

Note:
This will likely break things. So don't merge it until we have more test coverage.

[huchenlei]

Closing as conflict with #109