root: return to lab when git fails handling flags

When `lab` doesn't recognize a flag to its root command, e.g. `lab --v`,
it passes to git in the hope the user wants it to passthrough. However,
when git also fails to handle the flag, it prints a failure message and
its usage message, which is quite confusing, since the user is issuing
`lab`.

This patch checks the return from git before printing anything and in
case it's a failure, let `lab` return its usage message instead of git's.

Signed-off-by: Bruno Meneguele <bmeneg@redhat.com>

cmd/root.go

@@ -221,7 +221,9 @@ func Execute() {
 		return
 	}
 
-	// allow flags to the root cmd to be passed through. Technically we'll drop any exit code info which isn't ideal.
+	// allow flags to the root cmd to be passed through.
+	// Error msgs and codes generated by git are thrown away and the execution
+	// flow returns to lab to generate the error.
 	// TODO: remove for 1.0 when we stop wrapping git
 	if cmd.Use == RootCmd.Use && len(os.Args) > 1 {
 		var knownFlag bool
@@ -230,10 +232,19 @@ func Execute() {
 				knownFlag = true
 			}
 		}
+
+		// Pass unknown flags to git, if it also doesn't handle it, let lab
+		// handle the exit msg (help) and code.
 		if !knownFlag {
 			log.Println("Warning: lab's git passthrough command support will be removed in a later release.")
-			git.New(os.Args[1:]...).Run()
-			return
+			gitCmd := git.New(os.Args[1:]...)
+			gitCmd.Stderr = nil
+			gitCmd.Stdout = nil
+			out, err := gitCmd.CombinedOutput()
+			if err == nil {
+				fmt.Println(string(out))
+				return
+			}
 		}
 	}