Use utf8::reencode_string from git

This replaces the iconv-support in cgit with similar functions already
existing in git.

Signed-off-by: Lars Hjemli <hjemli@gmai.com>

cgit.h

@@ -16,6 +16,7 @@
 #include <log-tree.h>
 #include <archive.h>
 #include <xdiff/xdiff.h>
+#include <utf8.h>
 
 
 /*