OwnersDb: Use Repository#exactRef instead of the deprecated Repository#getRef

Change-Id: I864ae2299ccabc1ff150d91727653d85bacbae96
Signed-off-by: Edwin Kempin <ekempin@google.com>
diff --git a/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersDb.java b/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersDb.java
index 9bb11a1..2c0c704 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersDb.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersDb.java
@@ -99,7 +99,7 @@
         }
       }
       try {
-        revision = repository.getRef(branch).getObjectId().getName();
+        revision = repository.exactRef(branch).getObjectId().getName();
       } catch (Exception e) {
         log.error("Fail to get branch revision for " + Config.getChangeId(changeData), e);
       }