string-util: use GREEDY_ALLOC_ROUND_UP() in strextend()

This uses GREEDY_ALLOC_ROUND_UP() to grow the allocation size
exponentially. This should speed allocation loops up a bit, given
that we often call strextend() repeatedly in a loop on the same
buffer.

src/basic/string-util.c

@@ -830,7 +830,7 @@ char *strextend_with_separator_internal(char **x, const char *separator, ...) {
 
         need_separator = !isempty(*x);
 
-        nr = realloc(*x, l+1);
+        nr = realloc(*x, GREEDY_ALLOC_ROUND_UP(l+1));
         if (!nr)
                 return NULL;