ArchiveFormat: Suppress ImmutableEnumChecker warning

ArchiveCommand.Format is effectively immutable, and it's a JGit
class that we don't control here, so just suppress the warning.

Change-Id: I4b92ff9e3bde9fab9bf5a1b2ce46962db227b2d8

java/com/google/gitiles/ArchiveFormat.java

diff --git a/java/com/google/gitiles/ArchiveFormat.java b/java/com/google/gitiles/ArchiveFormat.java
index 76c4efc..a8bab0f 100644
--- a/java/com/google/gitiles/ArchiveFormat.java
+++ b/java/com/google/gitiles/ArchiveFormat.java
@@ -53,6 +53,7 @@
     }
   }
 
+  @SuppressWarnings("ImmutableEnumChecker") // ArchiveCommand.Format is effectively immutable.
   private final ArchiveCommand.Format<?> format;
   private final String mimeType;