[auth] Allow hub used in Actions by specifying GITHUB_USER

When supplied with GITHUB_TOKEN, hub will attempt to request the `user`
API resource which isn't available in the context of Actions.

To work around this, scripts may now set GITHUB_USER to avoid hitting
the unavailable API resource.

features/authentication.feature

@@ -261,9 +261,34 @@ Feature: OAuth authentication
     And the stderr should contain exactly:
       """
       Error getting current user: Forbidden (HTTP 403)
-      Resource not accessible by integration\n
+      Resource not accessible by integration
+      You must specify GITHUB_USER via environment variable.\n
       """
 
+  Scenario: Credentials from GITHUB_TOKEN and GITHUB_USER
+    Given I am in "git://github.com/monalisa/playground.git" git repo
+    Given the GitHub API server:
+      """
+      get('/user') {
+        status 403
+        json :message => "Resource not accessible by integration",
+             :documentation_url => "https://developer.github.com/v3/users/#get-the-authenticated-user"
+      }
+      get('/repos/monalisa/playground/releases') {
+        halt 401 unless request.env["HTTP_AUTHORIZATION"] == "token OTOKEN"
+        json [
+          { tag_name: 'v1.2.0',
+          }
+        ]
+      }
+      """
+    Given $GITHUB_TOKEN is "OTOKEN"
+    Given $GITHUB_USER is "hubot"
+    When I successfully run `hub release show v1.2.0`
+    Then the output should not contain "github.com password"
+    And the output should not contain "github.com username"
+    And the file "../home/.config/hub" should not exist
+
   Scenario: Credentials from GITHUB_TOKEN override those from config file
     Given I am "mislav" on github.com with OAuth token "OTOKEN"
     Given the GitHub API server:

github/client.go

@@ -1044,6 +1044,9 @@ func FormatError(action string, err error) (ee error) {
 			errorMessage = strings.Join(errorSentences, "\n")
 		} else {
 			errorMessage = e.Message
+			if action == "getting current user" && e.Message == "Resource not accessible by integration" {
+				errorMessage = errorMessage + "\nYou must specify GITHUB_USER via environment variable."
+			}
 		}
 
 		if errorMessage != "" {

github/config.go

@@ -87,11 +87,17 @@ func (c *Config) PromptForHost(host string) (h *Host, err error) {
 		}
 	}
 
-	currentUser, err := client.CurrentUser()
-	if err != nil {
-		return
+	userFromEnv := os.Getenv("GITHUB_USER")
+	if tokenFromEnv && userFromEnv != "" {
+		h.User = userFromEnv
+	} else {
+		var currentUser *User
+		currentUser, err = client.CurrentUser()
+		if err != nil {
+			return
+		}
+		h.User = currentUser.Login
 	}
-	h.User = currentUser.Login
 
 	if !tokenFromEnv {
 		err = newConfigService().Save(configsFile(), c)