[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

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)
+}