Use MergedEvent to improve the merge commit detection (#503)

Even though PR is in "MERGED" state, the mergeCommit field can be empty
in the GitHub API response. Use the MergedEvent to get the merge commit
hash to complement the detection.

e2e_tests/mock_ghgql_server.go

@@ -10,7 +10,7 @@ import (
 
 const (
 	// These are ugly, but this is easy way to tell which query is being used.
-	prQuery = "query($after:String$baseRefName:String$first:Int!$headRefName:String$owner:String!$repo:String!$states:[PullRequestState!]){repository(owner: $owner, name: $repo){pullRequests(states: $states, headRefName: $headRefName, baseRefName: $baseRefName, first: $first, after: $after){nodes{id,number,headRefName,baseRefName,isDraft,permalink,state,title,body,mergeCommit{oid},timelineItems(last: 10, itemTypes: CLOSED_EVENT){nodes{... on ClosedEvent{closer{... on Commit{oid}}}}}},pageInfo{endCursor,hasNextPage,hasPreviousPage,startCursor}}}}"
+	prQuery = "query($after:String$baseRefName:String$first:Int!$headRefName:String$owner:String!$repo:String!$states:[PullRequestState!]){repository(owner: $owner, name: $repo){pullRequests(states: $states, headRefName: $headRefName, baseRefName: $baseRefName, first: $first, after: $after){nodes{id,number,headRefName,baseRefName,isDraft,permalink,state,title,body,mergeCommit{oid},timelineItems(last: 10, itemTypes: [CLOSED_EVENT, MERGED_EVENT]){nodes{... on ClosedEvent{closer{... on Commit{oid}}},... on MergedEvent{commit{oid}}}}},pageInfo{endCursor,hasNextPage,hasPreviousPage,startCursor}}}}"
 )
 
 func RunMockGitHubServer(t *testing.T) *mockGitHubServer {

internal/gh/pullrequest.go

@@ -30,8 +30,13 @@ type PullRequest struct {
 					} `graphql:"... on Commit"`
 				}
 			} `graphql:"... on ClosedEvent"`
+			MergedEvent struct {
+				Commit struct {
+					Oid string
+				}
+			} `graphql:"... on MergedEvent"`
 		}
-	} `graphql:"timelineItems(last: 10, itemTypes: CLOSED_EVENT)"`
+	} `graphql:"timelineItems(last: 10, itemTypes: [CLOSED_EVENT, MERGED_EVENT])"`
 }
 
 func (p *PullRequest) HeadBranchName() string {
@@ -49,10 +54,19 @@ func (p *PullRequest) BaseBranchName() string {
 func (p *PullRequest) GetMergeCommit() string {
 	if p.State == githubv4.PullRequestStateOpen {
 		return ""
-	} else if p.State == githubv4.PullRequestStateMerged {
+	} else if p.State == githubv4.PullRequestStateMerged && p.PRIVATE_MergeCommit.Oid != "" {
 		return p.PRIVATE_MergeCommit.Oid
-	} else if p.State == githubv4.PullRequestStateClosed && len(p.PRIVATE_TimelineItems.Nodes) != 0 {
-		return p.PRIVATE_TimelineItems.Nodes[0].ClosedEvent.Closer.Commit.Oid
+	}
+	// The timeline is in chronological order, so we can iterate backwards to find the latest
+	// one.
+	for i := len(p.PRIVATE_TimelineItems.Nodes) - 1; i >= 0; i-- {
+		item := p.PRIVATE_TimelineItems.Nodes[i]
+		if item.ClosedEvent.Closer.Commit.Oid != "" {
+			return item.ClosedEvent.Closer.Commit.Oid
+		}
+		if item.MergedEvent.Commit.Oid != "" {
+			return item.MergedEvent.Commit.Oid
+		}
 	}
 	return ""
 }