MergeOp: Pre-open all OpenRepos

The openRepo method may throw NoSuchProjectException, which should
fail the merge process and abandon all open changes. Better to do this
in advance rather than lazily after some side effects may have already
happened.

Change-Id: Iba219f83de65f19a329088ff60366e34eebe569e

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

@@ -1830,8 +1830,6 @@ public class ReceiveCommits {
     try (MergeOp op = mergeOpProvider.get()) {
       op.merge(db, rsrc.getChange(),
           changeCtl.getUser().asIdentifiedUser(), false);
-    } catch (NoSuchChangeException e) {
-      throw new OrmException(e);
     }
     addMessage("");
     Change c = db.changes().get(rsrc.getChange().getId());