contributors.yml: use git shortlog

[natalialuzuriaga]

Is your feature request related to a problem? Please describe.
For public repos, uses GitHub API to retrieve contributor info
No way to retrieve contributor information for internal / non GitHub repos

Describe the solution you'd like
For internal repos / non GitHub repos, use git shortlog: contrib-resolution-rough.sh that @IsaacMilarky wrote

Describe alternatives you've considered

Additional context