commit-tree: cope with different ways "utf-8" can be spelled.

People can spell config.commitencoding differently from what we
internally have ("utf-8") to mean UTF-8.  Try to accept them and
treat them equally.

Signed-off-by: Junio C Hamano <junkio@cox.net>

utf8.c

@@ -277,6 +277,15 @@ void print_wrapped_text(const char *text, int indent, int indent2, int width)
 	}
 }
 
+int is_encoding_utf8(const char *name)
+{
+	if (!name)
+		return 1;
+	if (!strcasecmp(name, "utf-8") || !strcasecmp(name, "utf8"))
+		return 1;
+	return 0;
+}
+
 /*
  * Given a buffer and its encoding, return it re-encoded
  * with iconv.  If the conversion fails, returns NULL.