Add Change.Id#toRefPrefix() and use from PatchSet.Id#toRefName().

Presize the StringBuilder; the default size for new StringBuilder() is
16, but "refs/changes/01/1/1" is 19 characters, guaranteeing a wasted
allocation.

Change-Id: I1ac3e5ff40b4994e7b05979e9bb65c4e6c59d7a9

gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/client/Change.java

@@ -125,6 +125,23 @@ public final class Change {
       id = newValue;
     }
 
+    public String toRefPrefix() {
+      return refPrefixBuilder().toString();
+    }
+
+    StringBuilder refPrefixBuilder() {
+      StringBuilder r = new StringBuilder(32)
+         .append(REFS_CHANGES);
+      int m = id % 100;
+      if (m < 10) {
+        r.append('0');
+      }
+      return r.append(m)
+          .append('/')
+          .append(id)
+          .append('/');
+    }
+
     /** Parse a Change.Id out of a string representation. */
     public static Id parse(final String str) {
       final Id r = new Id();

gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/client/PatchSet.java

@@ -14,8 +14,6 @@
 
 package com.google.gerrit.reviewdb.client;
 
-import static com.google.gerrit.reviewdb.client.RefNames.REFS_CHANGES;
-
 import com.google.gwtorm.client.Column;
 import com.google.gwtorm.client.IntKey;
 
@@ -109,19 +107,9 @@ public final class PatchSet {
     }
 
     public String toRefName() {
-      StringBuilder r = new StringBuilder();
-      r.append(REFS_CHANGES);
-      int change = changeId.get();
-      int m = change % 100;
-      if (m < 10) {
-        r.append('0');
-      }
-      r.append(m);
-      r.append('/');
-      r.append(change);
-      r.append('/');
-      r.append(patchSetId);
-      return r.toString();
+      return changeId.refPrefixBuilder()
+          .append(patchSetId)
+          .toString();
     }
 
     /** Parse a PatchSet.Id out of a string representation. */

gerrit-reviewdb/src/test/java/com/google/gerrit/reviewdb/client/ChangeTest.java

@@ -71,6 +71,14 @@ public class ChangeTest {
     assertNotRef("refs/changes/01/1/1/meta");
   }
 
+  @Test
+  public void toRefPrefix() {
+    assertThat(new Change.Id(1).toRefPrefix())
+        .isEqualTo("refs/changes/01/1/");
+    assertThat(new Change.Id(1234).toRefPrefix())
+        .isEqualTo("refs/changes/34/1234/");
+  }
+
   private static void assertRef(int changeId, String refName) {
     assertThat(Change.Id.fromRef(refName)).isEqualTo(new Change.Id(changeId));
   }

gerrit-reviewdb/src/test/java/com/google/gerrit/reviewdb/client/PatchSetTest.java

@@ -83,6 +83,14 @@ public class PatchSetTest {
     assertThat(joinGroups(ImmutableList.of("", "cd"))).isEqualTo(",cd");
   }
 
+  @Test
+  public void testToRefName() {
+    assertThat(new PatchSet.Id(new Change.Id(1), 23).toRefName())
+        .isEqualTo("refs/changes/01/1/23");
+    assertThat(new PatchSet.Id(new Change.Id(1234), 5).toRefName())
+        .isEqualTo("refs/changes/34/1234/5");
+  }
+
   private static void assertRef(int changeId, int psId, String refName) {
     assertThat(PatchSet.isChangeRef(refName)).isTrue();
     assertThat(PatchSet.Id.fromRef(refName))