Must not modify the_index.cache as it may be passed to realloc at some point.

Must not modify the_index.cache as it may be passed to realloc at some point.

The index cache is not static, growing as new entries are added. If
entries are added after prune_cache is called, cache will no longer
point at the base of the allocation, and realloc will not be happy.

I verified that this was the only place in the current source which
modified any index_state.cache elements aside from the alloc/realloc
calls in read-cache by changing the type of the element to 'struct
cache_entry ** const cache' and recompiling.

A more efficient patch would create a separate 'cache_base' value to
track the allocation and then fix things up when reallocation was
necessary, instead of the brute-force memmove used here.

Signed-off-by: Junio C Hamano <gitster@pobox.com>