Improve commit message when updating subscribed git submodules

When a git submodule is updated in a project, the update is done with
a commit that has the title "Updated project" where 'project' is the
name of the project being updated.

Change it to "Updated git submodules" which is a bit more descriptive
of the actual change.

Change-Id: I4b54198c96bcbc7d6efdf7764704833751ecf5e5

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

diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/git/SubmoduleOp.java b/gerrit-server/src/main/java/com/google/gerrit/server/git/SubmoduleOp.java
index 773d665..ab18641 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/git/SubmoduleOp.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/git/SubmoduleOp.java
@@ -289,10 +289,7 @@
       throws SubmoduleException {
     PersonIdent author = null;
 
-    final StringBuilder msgbuf = new StringBuilder();
-    msgbuf.append("Updated ")
-      .append(subscriber.getParentKey().get())
-      .append('\n');
+    final StringBuilder msgbuf = new StringBuilder("Updated git submodules\n");
     Repository pdb = null;
     RevWalk recRw = null;