[FIXED JENKINS-19392] Simpler and more efficient to use String.intern to compact fingerprint data.

This should work across builds and even across jobs, and does not force us to load other builds.
(cherry picked from commit 2f59ffc4)

Conflicts:
	changelog.html

core/src/main/java/hudson/tasks/Fingerprinter.java

@@ -294,8 +294,7 @@ public class Fingerprinter extends Recorder implements Serializable, DependencyD
 
         public FingerprintAction(Run build, Map<String, String> record) {
             this.build = build;
-            this.record = PackedMap.of(record);
-            compact();
+            this.record = compact(record);
         }
 
         @Deprecated
@@ -306,9 +305,8 @@ public class Fingerprinter extends Recorder implements Serializable, DependencyD
         public void add(Map<String,String> moreRecords) {
             Map<String,String> r = new HashMap<String, String>(record);
             r.putAll(moreRecords);
-            record = PackedMap.of(r);
+            record = compact(r);
             ref = null;
-            compact();
         }
 
         public String getIconFileName() {
@@ -341,48 +339,20 @@ public class Fingerprinter extends Recorder implements Serializable, DependencyD
 
         @Override public void onLoad(Run<?,?> r) {
             build = r;
-            compact();
+            record = compact(record);
         }
 
         @Override public void onAttached(Run<?,?> r) {
             // for historical reasons this setup is done in the constructor instead
         }
 
-        private void compact() {
-            // share data structure with nearby builds, but to keep lazy loading efficient,
-            // don't go back the history forever.
-            if (rand.nextInt(2)!=0) {
-                Run pb = build.getPreviousBuild();
-                if (pb!=null) {
-                    FingerprintAction a = pb.getAction(FingerprintAction.class);
-                    if (a!=null)
-                        compact(a);
-                }
-            }
-        }
-
-        /**
-         * Reuse string instances from another {@link FingerprintAction} to reduce memory footprint.
-         */
-        protected void compact(FingerprintAction a) {
-            Map<String,String> intern = new HashMap<String, String>(); // string intern map
-            for (Entry<String, String> e : a.record.entrySet()) {
-                intern.put(e.getKey(),e.getKey());
-                intern.put(e.getValue(),e.getValue());
-            }
-
-            Map<String,String> b = new HashMap<String, String>();
+        /** Share data structure with other builds, mainly those of the same job. */
+        private PackedMap<String,String> compact(Map<String,String> record) {
+            Map<String,String> b = new HashMap<String,String>();
             for (Entry<String,String> e : record.entrySet()) {
-                String k = intern.get(e.getKey());
-                if (k==null)    k = e.getKey();
-
-                String v = intern.get(e.getValue());
-                if (v==null)    v = e.getValue();
-
-                b.put(k,v);
+                b.put(e.getKey().intern(), e.getValue().intern());
             }
-
-            record = PackedMap.of(b);
+            return PackedMap.of(b);
         }
 
         /**