Fix escaping of strings in yaml lists in ./mill init (#6798)

fixes #6795

Author: lihaoyi

libs/init/buildgen/src/mill/main/buildgen/BuildGenYaml.scala

@@ -379,15 +379,15 @@ object BuildGenYaml extends BuildGen {
     if (flatOpts.isEmpty) Nil
     else {
       val appendTag = if (values.appendSuper) "!append " else ""
-      Seq(s"$name: $appendTag${renderYamlStringList(flatOpts)}")
+      Seq(s"$name: $appendTag${renderYamlStringList(flatOpts.map(yamlEscapeStringInList))}")
     }
   }
 
   private def renderYamlStringListValuesPlain(name: String, values: Values[String]): Seq[String] = {
     if (values.base.isEmpty) Nil
     else {
       val appendTag = if (values.appendSuper) "!append " else ""
-      Seq(s"$name: $appendTag${renderYamlStringList(values.base)}")
+      Seq(s"$name: $appendTag${renderYamlStringList(values.base.map(yamlEscapeStringInList))}")
     }
   }
 

libs/init/gradle/test/resources/expected/gradle-6-0/build.mill.yaml

@@ -4,7 +4,7 @@ extends: MavenModule
 mvnDeps:
 - com.google.guava:guava:28.0-jre
 javacOptions: [-source, 11, -target, 11]
-repositories: [https://jcenter.bintray.com/]
+repositories: ["https://jcenter.bintray.com/"]
 
 object test:
   extends: [MavenTests, TestModule.TestNg]