Reject pushing to invalid reference names

If a reference name is not valid according to Git's rules for
the allowable syntax, or it contains "//" because a typo on the
client side introduced an unnecessary slash, reject the command
with an error.

Bug: issue 466
Change-Id: I89d8137a89225bc31813f5870d75568e5080b059
Signed-off-by: Shawn O. Pearce <sop@google.com>

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

@@ -404,6 +404,12 @@ public class ReceiveCommits implements PreReceiveHook, PostReceiveHook {
         continue;
       }
 
+      if (!Repository.isValidRefName(cmd.getRefName())
+          || cmd.getRefName().contains("//")) {
+        reject(cmd, "not valid ref");
+        continue;
+      }
+
       if (cmd.getRefName().startsWith(NEW_CHANGE)) {
         parseNewChangeCommand(cmd);
         continue;