read_tree(): respect max_allowed_tree_depth

read_tree(): respect max_allowed_tree_depth

The read_tree() function reads trees recursively (via its read_tree_at()
helper). This can cause it to run out of stack space on very deep trees.
Let's teach it about the new core.maxTreeDepth option.

The easiest way to demonstrate this is via "ls-tree -r", which the test
covers. Note that I needed a tree depth of ~30k to trigger a segfault on
my Linux system, not the 4100 used by our "big" test in t6700. However,
that test still tells us what we want: that the default 4096 limit is
enough to prevent segfaults on all platforms. We could bump it, but that
increases the cost of the test setup for little gain.

As an interesting side-note: when I originally wrote this patch about 4
years ago, I needed a depth of ~50k to segfault. But porting it forward,
the number is much lower. Seemingly little things like cf0983213c (hash:
add an algo member to struct object_id, 2021-04-26) take it from 32,722
to 29,080.

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