Add patch set description

With this change, the description field is properly populated by the %m
flag and set in ReviewDB and NoteDB, and is set in the ChangeJSON
received in a GetChange call. The test for pushing a change with a
message was also updated to check for the description field being set in
RevisionInfo.

To be included in descendant changes:
 - Handling cases where PSUtil.insert is called that aren't change
   creations or updates (cherrypick, etc)
 - Implementation of an API endpoint for setting/deleting the value
 - Docs

Feature: Issue 4544
Change-Id: I1c4d817d1d5e263b0cd42bf9ea874808aac00582

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

@@ -194,6 +194,16 @@ public final class PatchSet {
   @Column(id = 8, notNull = false, length = Integer.MAX_VALUE)
   protected String pushCertificate;
 
+  /**
+   * Optional user-supplied description for this patch set.
+   * <p>
+   * When this field is null, the description was never set on the patch set.
+   * When this field is an empty string, the description was set and later
+   * cleared.
+   */
+  @Column(id = 9, notNull = false, length = Integer.MAX_VALUE)
+  protected String description;
+
   protected PatchSet() {
   }
 
@@ -209,6 +219,7 @@ public final class PatchSet {
     this.draft = src.draft;
     this.groups = src.groups;
     this.pushCertificate = src.pushCertificate;
+    this.description = src.description;
   }
 
   public PatchSet.Id getId() {
@@ -277,6 +288,14 @@ public final class PatchSet {
     pushCertificate = cert;
   }
 
+  public String getDescription() {
+    return description;
+  }
+
+  public void setDescription(String description) {
+    this.description = description;
+  }
+
   @Override
   public String toString() {
     return "[PatchSet " + getId().toString() + "]";

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

@@ -54,6 +54,9 @@ public final class PatchSetInfo {
   /** SHA-1 of commit */
   protected String revId;
 
+  /** Optional user-supplied description for the patch set. */
+  protected String description;
+
   protected PatchSetInfo() {
   }
 
@@ -116,4 +119,12 @@ public final class PatchSetInfo {
   public String getRevId() {
     return revId;
   }
+
+  public void setDescription(String description) {
+    this.description = description;
+  }
+
+  public String getDescription() {
+    return description;
+  }
 }