Add a fallback to git diff command to get diff of a GitHub pull request on doghouse

[massongit]

• Updated Unreleased section in CHANGELOG or it's not notable changes.

Fix #1696 (comment)

Similar to #1697 and #1714, I add a fallback to git diff command to get diff of a GitHub pull request on doghouse.

reviewdog/cmd/reviewdog/main.go

Lines 302 to 307 in 8e405ab

	case "github-check":
	return runDoghouse(ctx, r, w, opt, isProject, false, false)
	case "github-pr-check":
	return runDoghouse(ctx, r, w, opt, isProject, true, false)
	case "github-pr-annotations":
	return runDoghouse(ctx, r, w, opt, isProject, true, true)

This will enable fallback even if the reporter is github-check, github-pr-check or github-pr-annotations.

Note that this only runs if git command is installed.