Set core.askPass config for better auth-fail error

Instead of "No such device or address", which is pretty obtuse, we now
get "Authentication failed".

main.go

@@ -2072,6 +2072,10 @@ func (git *repoSync) SetupDefaultGitConfigs(ctx context.Context) error {
 		// How to manage credentials (for those modes that need it).
 		key: "credential.helper",
 		val: "cache --timeout 3600",
+	}, {
+		// Never prompt for a password.
+		key: "core.askPass",
+		val: "true",
 	}, {
 		// Mark repos as safe (avoid a "dubious ownership" error).
 		key: "safe.directory",