tree-diff: fix leak when not HAVE_ALLOCA_H

b8ba412bf7 (tree-diff: avoid alloca for large allocations, 2016-06-07)
adds a way to route some bigger allocations out of the stack and free
them through the addition of two conveniently named macros, but leaves
the calls to free the xalloca part, which could be also in the heap,
if the system doesn't HAVE_ALLOCA_H (ex: macOS and other BSD).

Add the missing free call, xalloca_free(), which is a noop if we
allocated memory in the stack frame, but a real free() if we
allocated in the heap instead, and while at it, change the expression
to match in both macros for ease of readability.

This avoids a leak reported by LSAN while running t0000 but that
wouldn't fail the test (which is fixed in the next patch):

  SUMMARY: LeakSanitizer: 1034 byte(s) leaked in 15 allocation(s).

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
