Ensure reproducible output from generate task (google#119)

The descriptors file generated by protoc depends on the order of the source files passed to it. This change makes sure that the input files passed to it are always in a consistent order, to ensure generated descriptors have a canonical representation to avoid triggering unnecessary rebuilds downstream.

src/main/groovy/com/google/protobuf/gradle/GenerateProtoTask.groovy

@@ -361,7 +361,9 @@ public class GenerateProtoTask extends DefaultTask {
     Preconditions.checkState(state == State.FINALIZED, 'doneConfig() has not been called')
 
     ToolsLocator tools = project.protobuf.tools
-    Set<File> protoFiles = inputs.sourceFiles.files
+    // Sort to ensure generated descriptors have a canonical representation
+    // to avoid triggering unnecessary rebuilds downstream
+    List<File> protoFiles = inputs.sourceFiles.files.sort()
 
     [builtins, plugins]*.each { plugin ->
       File outputDir = new File(getOutputDir(plugin))