x/build/maintner: GitHubIssue.ID field is never populated

[dmitshur]

Similarly to #28745, the ID field of a GitHubIssue is always zero:

corpus, err := godata.Get(context.Background())
if err != nil {
	log.Fatal(err)
}
num := 0
corpus.GitHub().ForeachRepo(func(gr *maintner.GitHubRepo) error {
	return gr.ForeachIssue(func(gi *maintner.GitHubIssue) error {
		if gi.ID != 0 {
			num++
		}
		return nil
	})
})
fmt.Printf("%d GitHub issues with non-zero IDs.\n", num)

// Output:
// 0 GitHub issues with non-zero IDs.

The field was added in CL 37888, but GithubIssueMutation.ID is not set anywhere. It should probably be set in the githubIssueDiffer.Diff method, with care taken that it's populated for both new and existing issues (it would be unhelpful to populate it only for new issues, as no one could rely on the field then).

An alternative fix is to document that it's always zero and/or to remove it.

This isn't high priority because the Number field is usable as a workaround; filing so I can look it up. (Fixing this is made harder by #37603.)

[gopherbot]

Change https://golang.org/cl/236260 mentions this issue: cmd/releasebot: remove okay-after-beta1 label after beta1 is released