make show-index a builtin

make show-index a builtin

The git-show-index command is built as its own separate
program. There's really no good reason for this, and it
means we waste extra space on disk (and CPU time running the
linker). Let's fold it in to the main binary as a builtin.

The history here is actually a bit amusing. The program
itself is mostly self-contained, and doesn't even use our
normal pack index code. In a5031214c4 (slim down "git
show-index", 2010-01-21), we even stopped using xmalloc() so
that it could avoid libgit.a entirely. But then 040a655116
(cleanup: use internal memory allocation wrapper functions
everywhere, 2011-10-06) switched that back to xmalloc, which
later become ALLOC_ARRAY().

Making it a builtin should give us the best of both worlds:
no wasted space and no need to avoid the usual patterns.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>