unpack-trees: plug minor memory leak

unpack-trees: plug minor memory leak

The allocations made by unpack_nondirectories() using create_ce_entry()
are never freed.

In the non-merge case, we duplicate them using add_entry() and later
only look at the first allocated element (src[0]), perhaps even only
by mistake.  Split out the actual addition from add_entry() into the
new helper do_add_entry() and call this non-duplicating function
instead of add_entry() to avoid the leak.

Valgrind reports this for the command "git archive v1.7.9" without
the patch:

  ==13372== LEAK SUMMARY:
  ==13372==    definitely lost: 230,986 bytes in 2,325 blocks
  ==13372==    indirectly lost: 0 bytes in 0 blocks
  ==13372==      possibly lost: 98 bytes in 1 blocks
  ==13372==    still reachable: 2,259,198 bytes in 3,243 blocks
  ==13372==         suppressed: 0 bytes in 0 blocks

And with the patch applied:

  ==13375== LEAK SUMMARY:
  ==13375==    definitely lost: 65 bytes in 1 blocks
  ==13375==    indirectly lost: 0 bytes in 0 blocks
  ==13375==      possibly lost: 0 bytes in 0 blocks
  ==13375==    still reachable: 2,364,417 bytes in 3,245 blocks
  ==13375==         suppressed: 0 bytes in 0 blocks

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>