slim down "git show-index"

slim down "git show-index"

As the documentation says, this is primarily for debugging, and
in the longer term we should rename it to test-show-index or something.

In the meantime, just avoid xmalloc (which slurps in the rest of git), and
separating out the trivial hex functions into "hex.o".

This results in

  [torvalds@nehalem git]$ size git-show-index
       text    data     bss     dec     hex filename
     222818    2276  112688  337782   52776 git-show-index (before)
       5696     624    1264    7584    1da0 git-show-index (after)

which is a whole lot better.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>