[Feature] [API] Get a single commit

[6543]

doc ref: https://developer.github.com/v3/repos/commits/#get-a-single-commit

gitea/modules/convert/convert.go

Line 318 in 32fb813

// TODO: Add the /commits API endpoint and use it here (https://developer.github.com/v3/repos/commits/#get-a-single-commit)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. I am here to help clear issues left open even if solved or waiting for more insight. This issue will be closed if no further activity occurs during the next 2 weeks. If the issue is still valid just add a comment to keep it alive. Thank you for your contributions.

[6543]

I'll take this