Don't treat external files as immutable

Fixes #352.

RELNOTES: Files in external repositories are now treated as mutable, which will make the correctness guarantees of using external repositories stronger (existent), but may cause performance penalties.

--
MOS_MIGRATED_REVID=109676408
diff --git a/src/test/java/com/google/devtools/build/lib/skyframe/FilesetEntryFunctionTest.java b/src/test/java/com/google/devtools/build/lib/skyframe/FilesetEntryFunctionTest.java
index 17eb2a1..302bb2b 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/FilesetEntryFunctionTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/FilesetEntryFunctionTest.java
@@ -85,12 +85,12 @@
         new PathPackageLocator(outputBase, ImmutableList.of(rootDirectory)));
     AtomicReference<ImmutableSet<PackageIdentifier>> deletedPackages =
         new AtomicReference<>(ImmutableSet.<PackageIdentifier>of());
-    ExternalFilesHelper externalFilesHelper = new ExternalFilesHelper(pkgLocator);
+    ExternalFilesHelper externalFilesHelper = new ExternalFilesHelper(pkgLocator, false);
 
     Map<SkyFunctionName, SkyFunction> skyFunctions = new HashMap<>();
 
     skyFunctions.put(SkyFunctions.FILE_STATE, new FileStateFunction(tsgm, externalFilesHelper));
-    skyFunctions.put(SkyFunctions.FILE, new FileFunction(pkgLocator, tsgm, externalFilesHelper));
+    skyFunctions.put(SkyFunctions.FILE, new FileFunction(pkgLocator));
     skyFunctions.put(SkyFunctions.DIRECTORY_LISTING, new DirectoryListingFunction());
     skyFunctions.put(
         SkyFunctions.DIRECTORY_LISTING_STATE,