Print color legend in the help for outdated instead of every invocation

Source/carthage/Formatting.swift

@@ -85,12 +85,20 @@ public struct ColorOptions: OptionsProtocol {
 	}
 
 	public static func evaluate(_ mode: CommandMode) -> Result<ColorOptions, CommandantError<CarthageError>> {
+		return self.evaluate(mode, additionalUsage: nil)
+	}
+
+	public static func evaluate(_ mode: CommandMode, additionalUsage: String? = nil) -> Result<ColorOptions, CommandantError<CarthageError>> {
+		var usage = "whether to apply color and terminal formatting (one of 'auto', 'always', or 'never')"
+		if let additionalUsage = additionalUsage {
+			usage += "\n" + additionalUsage
+		}
 		return curry(self.init)
 			<*> mode <| Option(
 				key: "color",
 				defaultValue: ColorArgument.auto,
-				usage: "whether to apply color and terminal formatting (one of 'auto', 'always', or 'never')"
-			)
+				usage: usage
+		)
 	}
 }
 
@@ -114,20 +122,4 @@ extension ColorOptions.Formatting {
 		}
 		return Color.Wrap(foreground: update.color).wrap
 	}
-
-	public var legendForOutdatedCommand: String? {
-		guard self.isColorful else { return nil }
-
-		let header = "Legend — <color> • «what happens when you run `carthage update`»:\n"
-		return [OutdatedCommand.UpdateType.newest, .newer, .ineligible].reduce(into: header) {
-			let (color, explanation) = ($1.color, $1.explanation)
-			let tabs = String(
-				repeating: "\t",
-				count: color == .yellow || color == .magenta ? 1 : 2
-			)
-			let colorDescription = Color.Wrap(foreground: color)
-				.wrap("<" + String(describing: color) + ">")
-			$0.append(colorDescription + tabs + "• " + explanation + "\n")
-		}
-	}
 }

Source/carthage/Outdated.swift

@@ -33,6 +33,18 @@ public struct OutdatedCommand: CommandProtocol {
 				return "Will not be updated because of the specified version in Cartfile."
 			}
 		}
+
+		static var legend: String {
+			let header = "Legend — <color> • «what happens when you run `carthage update`»:\n"
+			return header + [UpdateType.newest, .newer, .ineligible].map {
+				let (color, explanation) = ($0.color, $0.explanation)
+				let tabs = String(
+					repeating: "\t",
+					count: color == .yellow || color == .magenta ? 1 : 2
+				)
+				return "<" + String(describing: color) + ">" + tabs + "• " + explanation
+			}.joined(separator: "\n")
+		}
 	}
 
 	public struct Options: OptionsProtocol {
@@ -53,7 +65,7 @@ public struct OutdatedCommand: CommandProtocol {
 				<*> mode <| Option(key: "use-ssh", defaultValue: false, usage: "use SSH for downloading GitHub repositories")
 				<*> mode <| Option(key: "verbose", defaultValue: false, usage: "include nested dependencies")
 				<*> mode <| Option(key: "xcode-warnings", defaultValue: false, usage: "output Xcode compatible warning messages")
-				<*> ColorOptions.evaluate(mode)
+				<*> ColorOptions.evaluate(mode, additionalUsage: UpdateType.legend)
 				<*> mode <| projectDirectoryOption
 		}
 
@@ -93,7 +105,6 @@ public struct OutdatedCommand: CommandProtocol {
 							carthage.println(formatting.projectName(project.name) + " " + versionSummary)
 						}
 					}
-					formatting.legendForOutdatedCommand.map(carthage.println)
 				} else {
 					carthage.println("All dependencies are up to date.")
 				}