Add a hidden flag which allows us to set the size of the legacy globbing thread pool.

--
MOS_MIGRATED_REVID=94236393
diff --git a/src/main/java/com/google/devtools/build/lib/pkgcache/PackageCacheOptions.java b/src/main/java/com/google/devtools/build/lib/pkgcache/PackageCacheOptions.java
index 6db7cf6..e57fe0d 100644
--- a/src/main/java/com/google/devtools/build/lib/pkgcache/PackageCacheOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/pkgcache/PackageCacheOptions.java
@@ -120,6 +120,12 @@
           + "'private').")
   public RuleVisibility defaultVisibility;
 
+  @Option(name = "legacy_globbing_threads",
+      defaultValue = "100",
+      category = "undocumented",
+      help = "Number of threads to use for glob evaluation.")
+  public int globbingThreads;
+
   @Option(name = "min_pkg_count_for_ct_node_eviction",
       defaultValue = "3700",
       // Why is the default value 3700? As of December 2013, a medium target loads about this many