Fix String.lines compilation problems on Java 11

Reported with #410 and
reproduced with https://travis-ci.com/github/rtyley/bfg-repo-cleaner/builds/208827661

More about the underlying issue here:

scala/bug#11125

bfg-library/src/main/scala/com/madgag/git/bfg/model/Commit.scala

@@ -66,10 +66,10 @@ object CommitNode {
 }
 
 case class CommitNode(author: PersonIdent, committer: PersonIdent, message: String, encoding: Charset = Constants.CHARSET) {
-  lazy val subject = message.lines.toStream.headOption
+  lazy val subject = message.linesIterator.toStream.headOption
   lazy val lastParagraphBreak = message.lastIndexOf("\n\n")
   lazy val messageWithoutFooters = if (footers.isEmpty) message else (message take lastParagraphBreak)
-  lazy val footers: List[Footer] = message.drop(lastParagraphBreak).lines.collect {
+  lazy val footers: List[Footer] = message.drop(lastParagraphBreak).linesIterator.collect {
     case Footer.FooterPattern(key, value) => Footer(key, value)
   }.toList