def call(env)
      if revision_file.exist?
        ok revision_file.read
      elsif git_repository?
        ok git_current_commit_sha
      else
        not_found 'Could not determine SHA'
      end
    end
def call(env)
      if revision_file.exist?
        ok revision_file.read
      elsif git_repository?
        ok git_current_commit_sha
      else
        not_found 'Could not determine SHA'
      end
    end
Public: Initialize a new GitSha app.

 root - The String path to application root.

 Returns the instance of GitSha.
 Public: Rack interface, run when the application is requested.

 env - The Hash environment of the incoming request.

 Returns an Array `[status, headers, [body]]` for Rack.