Commits that don't belong to a PR appeared in the `pull_request_commits` table

[Startrekzky]

Describe the bug

pull_request_commits records are incorrect. This pr has only 10 commits, but there're more than 10 records with pr.id = 'github:GithubPullRequest:514445706' in the table.

To Reproduce

Steps to reproduce the behavior:

1. Go to 'triggers' page in the config-ui
2. run
   [ [ { "plugin":"github", "options":{ "owner":"tikv", "repo":"tikv", "tasks":[ "collectPullRequestCommits", "convertPullRequestCommits" ] } } ] ]
3. Click on Trigger collection

Expected behavior

table.Pull request commits has the right number and pair of records after collection is finished.

[klesh]

[klesh]

I think the converter logic is incorrect:
https://github.com/merico-dev/lake/blob/v0.9.1/plugins/github/tasks/github_pull_request_commit_convertor.go
line 40 should be using githubPullRequestCommit.PullRequestId for domain id generation instead of pullRequestId

[klesh]

fixed