Add specialized copy methods to IncOptions class.

Add methods that allow one to set a new value to one of the fields of
IncOptions class. These methods are meant to be an alternative to
copy method that is hard to keep binary compatible when new fields are
added to the class.

Each copying method is related to one field of the class so when new
fields are added existing methods (and their signatures) are unaffected.

compile/inc/src/main/scala/sbt/inc/IncOptions.scala

@@ -49,8 +49,43 @@ final class IncOptions(
 	val newClassfileManager: () => ClassfileManager
 ) extends Product with Serializable {
 
+	def withTransitiveStep(transitiveStep: Int): IncOptions = {
+		new IncOptions(transitiveStep, recompileAllFraction, relationsDebug, apiDebug, apiDiffContextSize,
+			apiDumpDirectory, newClassfileManager)
+	}
+
+	def withRecompileAllFraction(recompileAllFraction: Double): IncOptions = {
+		new IncOptions(transitiveStep, recompileAllFraction, relationsDebug, apiDebug, apiDiffContextSize,
+			apiDumpDirectory, newClassfileManager)
+	}
+
+	def withRelationsDebug(relationsDebug: Boolean): IncOptions = {
+		new IncOptions(transitiveStep, recompileAllFraction, relationsDebug, apiDebug, apiDiffContextSize,
+			apiDumpDirectory, newClassfileManager)
+	}
+
+	def withApiDebug(apiDebug: Boolean): IncOptions = {
+		new IncOptions(transitiveStep, recompileAllFraction, relationsDebug, apiDebug, apiDiffContextSize,
+			apiDumpDirectory, newClassfileManager)
+	}
+
+	def withApiDiffContextSize(apiDiffContextSize: Int): IncOptions = {
+		new IncOptions(transitiveStep, recompileAllFraction, relationsDebug, apiDebug, apiDiffContextSize,
+			apiDumpDirectory, newClassfileManager)
+	}
+
+	def withApiDumpDirectory(apiDumpDirectory: Option[File]): IncOptions = {
+		new IncOptions(transitiveStep, recompileAllFraction, relationsDebug, apiDebug, apiDiffContextSize,
+			apiDumpDirectory, newClassfileManager)
+	}
+
+	def withNewClassfileManager(newClassfileManager: () => ClassfileManager): IncOptions = {
+		new IncOptions(transitiveStep, recompileAllFraction, relationsDebug, apiDebug, apiDiffContextSize,
+			apiDumpDirectory, newClassfileManager)
+	}
+
 	//- EXPANDED CASE CLASS METHOD BEGIN -//
-	@deprecated("Methods generated for case class will be removed in the future.", "0.13.2")
+	@deprecated("Use `with$nameOfTheField` copying methods instead.", "0.13.2")
 	def copy(transitiveStep: Int = this.transitiveStep, recompileAllFraction: Double = this.recompileAllFraction,
 			 relationsDebug: Boolean = this.relationsDebug, apiDebug: Boolean = this.apiDebug,
 			 apiDiffContextSize: Int = this.apiDiffContextSize,