Teach core object handling functions about gitlinks

Teach core object handling functions about gitlinks

This teaches the really fundamental core SHA1 object handling routines
about gitlinks.  We can compare trees with gitlinks in them (although we
can not actually generate patches for them yet - just raw git diffs),
and they show up as commits in "git ls-tree".

We also know to compare gitlinks as if they were directories (ie the
normal "sort as trees" rules apply).

[jc: amended a cut&paste error]

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>