Fix GraphQL URL for Github Enterprise

Fixes runatlantis#1009

server/events/vcs/github_client.go

@@ -61,7 +61,9 @@ func NewGithubClient(hostname string, user string, pass string, logger *logging.
 			return nil, errors.Wrapf(err, "Invalid github hostname trying to parse %s", baseURL)
 		}
 		client.BaseURL = base
-		graphqlURL = fmt.Sprintf("https://%s/graphql", hostname)
+		// Github Enterprise uses a slightly different endpoint format for GraphQL.
+		// https://developer.github.com/enterprise/2.20/v4/guides/forming-calls/#the-graphql-endpoint
+		graphqlURL = fmt.Sprintf("https://%s/api/graphql", hostname)
 		_, err = url.Parse(graphqlURL)
 		if err != nil {
 			return nil, errors.Wrapf(err, "Invalid GraphQL github hostname trying to parse %s", graphqlURL)

server/events/vcs/github_client_internal_test.go

@@ -31,4 +31,6 @@ func TestNewGithubClient_NonGithub(t *testing.T) {
 	client, err := NewGithubClient("example.com", "user", "pass", nil)
 	Ok(t, err)
 	Equals(t, "https://example.com/api/v3/", client.client.BaseURL.String())
+	// If possible in the future, test the GraphQL client's URL as well. But at the
+	// moment the shurcooL library doesn't expose it.
 }