Fixes SI-6268. Review by @paulp and @lrytz.

This reverts a refactoring from scala@963aabbeb4

MultiString settings would not properly write an unparse string that could be
reparsed, leading to failures when forking scalac in ant. Specifically, if a
setting was empty, it was getting added to the unparse string and causing
scalac to fail. This at least reverts to previous behavior (also more correct
for multiple values). Whatever we do here has to work with the @file style
argument reading and can't place empty options on the command line.

Also, This assumes there are tests around the behvior the REPL needs.

Note: The test is not automated yet, so unfortunately, we need to improve ANT testing capabilities to prevent regressons

Author: jsuereth

src/compiler/scala/tools/nsc/settings/MutableSettings.scala

@@ -536,7 +536,7 @@ class MutableSettings(val errorFn: String => Unit)
     }
     override def tryToSetColon(args: List[String]) = tryToSet(args)
     override def tryToSetFromPropertyValue(s: String) = tryToSet(s.trim.split(',').toList)
-    def unparse: List[String] = name :: value
+    def unparse: List[String] = value map (name + ":" + _)
 
     withHelpSyntax(name + ":<" + arg + ">")
   }

test/ant/test-basic/build.xml

@@ -0,0 +1,33 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<project name="test-simple" default="compile">
+  <description>Super simple test for Scala</description>
+
+  <target name="init">
+     <!-- Define project CLASSPATH. -->
+    <property name="base.dir" value="../../.."/>
+    <property name="pack.dir" value="${base.dir}/build/pack/"/>
+    <property name="build.dir" value="classes"/>
+    <property name="src.dir" value="src"/>
+    <property name="jvmargs" value=""/>
+    <path id="scala.classpath">
+      <fileset dir="${pack.dir}/lib/"> <include name="*.jar" /> </fileset>
+    </path>
+
+    <!-- Define scala compiler, scaladoc, etc command -->
+    <taskdef resource="scala/tools/ant/antlib.xml">
+      <classpath refid="scala.classpath" />
+    </taskdef>
+  </target>
+
+  <target name="compile" depends="init">
+    <mkdir dir="${build.dir}"/>
+
+    <scalac srcdir="${src.dir}" destdir="${build.dir}"
+            classpathref="scala.classpath" fork="true" target="jvm-1.5"
+            deprecation="no" addparams="-no-specialization"
+            jvmargs="${jvmargs} -XX:+UseConcMarkSweepGC">
+      <include name="**/*.scala"/>
+    </scalac>
+  </target>
+</project>