[PATCH] fix bug in read-cache.c which loses files when merging a tree

I noticed this when I tried a non-trivial scsi merge and checked the
results against BK.  The problem is that remove_entry_at() actually
decrements active_nr, so decrementing it in add_cache_entry() before
calling remove_entry_at() is a double decrement (hence we lose cache
entries at the end).