mem-pool: simplify alignment calculation

Use DIV_ROUND_UP in mem_pool_alloc() to round the allocation length to
the next multiple of GIT_MAX_ALIGNMENT instead of twiddling bits
explicitly.  This is shorter and clearer, to the point that we no longer
need the comment that explains what's being calculated.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mem-pool.c

@@ -89,9 +89,7 @@ void *mem_pool_alloc(struct mem_pool *pool, size_t len)
  struct mp_block *p = NULL;
  void *r;
 
- /* round up to a 'GIT_MAX_ALIGNMENT' alignment */
- if (len & (GIT_MAX_ALIGNMENT - 1))
- len += GIT_MAX_ALIGNMENT - (len & (GIT_MAX_ALIGNMENT - 1));
+ len = DIV_ROUND_UP(len, GIT_MAX_ALIGNMENT) * GIT_MAX_ALIGNMENT;
 
  if (pool->mp_block &&
  pool->mp_block->end - pool->mp_block->next_free >= len)