[BUG] Use correct SHA in `GetCommitPullRequest`

- The param wasn't `sha`, it was `ref`. Use this instead.
- Adds new integration tests.
- Resolves #4190
- Resolves #4025

(cherry picked from commit a8460bb132)

routers/api/v1/repo/commits.go

@@ -354,7 +354,7 @@ func GetCommitPullRequest(ctx *context.APIContext) {
 	//   "404":
 	//     "$ref": "#/responses/notFound"
 
-	pr, err := issues_model.GetPullRequestByMergedCommit(ctx, ctx.Repo.Repository.ID, ctx.Params(":sha"))
+	pr, err := issues_model.GetPullRequestByMergedCommit(ctx, ctx.Repo.Repository.ID, ctx.Params("ref"))
 	if err != nil {
 		if issues_model.IsErrPullRequestNotExist(err) {
 			ctx.Error(http.StatusNotFound, "GetPullRequestByMergedCommit", err)

tests/integration/api_repo_test.go

@@ -749,3 +749,17 @@ func TestAPIViewRepoObjectFormat(t *testing.T) {
 	DecodeJSON(t, resp, &repo)
 	assert.EqualValues(t, "sha1", repo.ObjectFormatName)
 }
+
+func TestAPIRepoCommitPull(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	var pr api.PullRequest
+	req := NewRequest(t, "GET", "/api/v1/repos/user2/repo1/commits/1a8823cd1a9549fde083f992f6b9b87a7ab74fb3/pull")
+	resp := MakeRequest(t, req, http.StatusOK)
+
+	DecodeJSON(t, resp, &pr)
+	assert.EqualValues(t, 1, pr.ID)
+
+	req = NewRequest(t, "GET", "/api/v1/repos/user2/repo1/commits/not-a-commit/pull")
+	MakeRequest(t, req, http.StatusNotFound)
+}