format_commit_message(): simplify calls to logmsg_reencode()

All the other callers of logmsg_reencode() pass return value of
get_commit_output_encoding() or get_log_output_encoding().  Teach
the function to optionally take NULL as a synonym to "" aka "no
conversion requested" so that we can simplify the only remaining
calling site.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

pretty.c

@@ -500,7 +500,7 @@ char *logmsg_reencode(const struct commit *commit,
 	char *encoding;
 	char *out;
 
-	if (!*output_encoding)
+	if (!output_encoding || !*output_encoding)
 		return NULL;
 	encoding = get_header(commit, "encoding");
 	use_encoding = encoding ? encoding : utf8;
@@ -1184,23 +1184,15 @@ void format_commit_message(const struct commit *commit,
 			   const struct pretty_print_context *pretty_ctx)
 {
 	struct format_commit_context context;
-	static const char utf8[] = "UTF-8";
 	const char *output_enc = pretty_ctx->output_encoding;
 
 	memset(&context, 0, sizeof(context));
 	context.commit = commit;
 	context.pretty_ctx = pretty_ctx;
 	context.wrap_start = sb->len;
-	context.message = commit->buffer;
-	if (output_enc) {
-		char *enc = get_header(commit, "encoding");
-		if (strcmp(enc ? enc : utf8, output_enc)) {
-			context.message = logmsg_reencode(commit, output_enc);
-			if (!context.message)
-				context.message = commit->buffer;
-		}
-		free(enc);
-	}
+	context.message = logmsg_reencode(commit, output_enc);
+	if (!context.message)
+		context.message = commit->buffer;
 
 	strbuf_expand(sb, format, format_commit_item, &context);
 	rewrap_message_tail(sb, &context, 0, 0, 0);