Bump dub to v1.39

With inheritable dub.selections.json support.

dub.json

@@ -13,7 +13,7 @@
     "mainSourceFile": "src/reggae/reggae_main.d",
     "preGenerateCommands": ["rdmd $PACKAGE_DIR/list_payload.d $PACKAGE_DIR"],
     "dependencies": {
-        "dub": "~>1.38.0"
+        "dub": "~>1.39.0"
     },
     "subConfigurations": {
         "dub": "library"

dub.selections.json

@@ -1,7 +1,7 @@
 {
 	"fileVersion": 1,
 	"versions": {
-		"dub": "1.38.0",
+		"dub": "1.39.0-beta.1",
 		"unit-threaded": "2.2.0"
 	}
 }

payload/reggae/options.d

@@ -242,8 +242,20 @@ struct Options {
             maybeDubDeps ~= _dubProjectFile;
 
             const selectionsJsonPath = buildPath(projectPath, "dub.selections.json");
-            if (selectionsJsonPath.exists)
+            if (selectionsJsonPath.exists) {
                 maybeDubDeps ~= selectionsJsonPath;
+            } else version(Have_dub) {
+                import dub.packagemanager: PackageManager;
+                import dub.internal.vibecompat.inet.path: NativePath;
+
+                () @trusted {
+                    auto projectRoot = NativePath(projectPath);
+                    auto pm = new PackageManager(projectRoot);
+                    const res = pm.readSelections(projectRoot);
+                    if (!res.isNull())
+                        maybeDubDeps ~= res.get().absolutePath.toNativeString();
+                }();
+            }
         }
 
         return ranFromPath ~ maybeReggaeFile ~ maybeReggaeFileDeps ~ maybeDubDeps ~ dependencies;