Reuse cached data out of delta base cache.

Reuse cached data out of delta base cache.

A malloc() + memcpy() will always be faster than mmap() +
malloc() + inflate().  If the data is already there it is
certainly better to copy it straight away.

With this patch below I can do 'git log drivers/scsi/ >
/dev/null' about 7% faster.  I bet it might be even more on
those platforms with bad mmap() support.

Signed-off-by: Nicolas Pitre <redacted>
Signed-off-by: Junio C Hamano <redacted>