Fix resource leak when changeMerge.test is true

The changeMerge feature leaks a Repository handle every time it
executes a test. For our installation of Gerrit, this leaks some
file handles, which can cause the JVM to run out of descriptors
and/or run out of native memory that back those objects.

A Repository, like a ReviewDb, must always be closed by the
caller after it is no longer required. A quick scan through
other call sites indicates other uses are in fact closing,
this was the only bug found.

Change-Id: If3fac76e82478a5bf32085d704842d88926ddd97

gerrit-server/src/main/java/com/google/gerrit/server/git/MergeOp.java

@@ -249,10 +249,12 @@ public class MergeOp {
       log.error("Test merge attempt for change: " + change.getId()
           + " failed", e);
     } finally {
+      if (repo != null) {
+        repo.close();
+      }
       if (db != null) {
         db.close();
       }
-      db = null;
     }
   }
 
@@ -292,8 +294,9 @@ public class MergeOp {
       if (repo != null) {
         repo.close();
       }
-      db.close();
-      db = null;
+      if (db != null) {
+        db.close();
+      }
     }
   }