MergeUtil: Pass Charset directly to MergeFormatter.formatMerge

Since [1] the variant of #formatMerge that takes the charset name as
a String is deprecated in favor of a new one that takes a Charset.

[1] https://git.eclipse.org/r/#/c/130188/

Change-Id: Idcb8982fdfa4939ebf60951fd5031fd9cc47cb0f
diff --git a/java/com/google/gerrit/server/git/MergeUtil.java b/java/com/google/gerrit/server/git/MergeUtil.java
index cc3b415..9cfca88 100644
--- a/java/com/google/gerrit/server/git/MergeUtil.java
+++ b/java/com/google/gerrit/server/git/MergeUtil.java
@@ -348,7 +348,7 @@
     for (Map.Entry<String, MergeResult<? extends Sequence>> entry : mergeResults.entrySet()) {
       MergeResult<? extends Sequence> p = entry.getValue();
       try (TemporaryBuffer buf = new TemporaryBuffer.LocalFile(null, 10 * 1024 * 1024)) {
-        fmt.formatMerge(buf, p, "BASE", oursNameFormatted, theirsNameFormatted, UTF_8.name());
+        fmt.formatMerge(buf, p, "BASE", oursNameFormatted, theirsNameFormatted, UTF_8);
         buf.close();
 
         try (InputStream in = buf.openInputStream()) {