fix(doctor): account for worktree context

[AndrewADev]

Make sure the doctor command can correctly recognize git repo in a worktree context.

Previously, running bight doctor from a worktree context lead to messages like this:

$ bight doctor
bight doctor:
  [fail] git repo: .git/hooks/ not found — are you in a git repo?
  [fail] config: failed to load .bight.yml — file does not exist
  [info] hook: not installed — run `bight install` to automate on checkout
  [warn] skipping env file and var checks — config could not be loaded
Error: one or more checks failed

Now it correctly recognizes that it is in a git repo:

$ bight doctor
bight doctor:
  [ok]   git repo detected
  [fail] config: failed to load .bight.yml — file does not exist
  [info] hook: not installed — run `bight install` to automate on checkout
  [warn] skipping env file and var checks — config could not be loaded
Error: one or more checks failed