diff-tree: fix crash when used with --remerge-diff

When using "git-diff-tree" to get the tree diff for merge commits with
the diff format set to `remerge`, a bug is triggered as shown below:

  $ git diff-tree -r --remerge-diff 363337e6eb
  363337e6eb
  BUG: log-tree.c:1006: did a remerge diff without remerge_objdir?!?

This bug is reported by `log-tree.c:do_remerge_diff`, where a bug check
added in commit 7b90ab467a (log: clean unneeded objects during log
--remerge-diff, 2022-02-02) detects the absence of `remerge_objdir` when
attempting to clean up temporary objects generated during the remerge
process.

After some further digging, I find that the remerge-related diff options
were introduced in db757e8b8d (show, log: provide a --remerge-diff
capability, 2022-02-02), which also affect the setup of `rev_info` for
"git-diff-tree", but were not accounted for in the original
implementation (inferred from the commit message).

Elijah Newren, the author of the remerge diff feature, notes that other
callers of `log-tree.c:log_tree_commit` (the only caller of
`log-tree.c:do_remerge_diff`) also exist, but:

  `builtin/am.c`: manually sets all flags; remerge_diff is not among them
  `sequencer.c`: manually sets all flags; remerge_diff is not among them

so `builtin/diff-tree.c` really is the only caller that was overlooked
when remerge-diff functionality was added.

This commit resolves the crash by adding `remerge_objdir` setup logic to
`builtin/diff-tree.c`, mirroring `builtin/log.c:cmd_log_walk_no_free`.
It also includes the necessary cleanup for `remerge_objdir`.

Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Xing Xin <xingxin.xx@bytedance.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4069-remerge-diff.sh

@@ -110,6 +110,41 @@ test_expect_success 'can filter out additional headers with pickaxe' '
 	test_must_be_empty actual
 '
 
+test_expect_success 'remerge-diff also works for git-diff-tree' '
+	# With a clean merge
+	git diff-tree -r -p --remerge-diff --no-commit-id bc_resolution >actual &&
+	test_must_be_empty actual &&
+
+	# With both a resolved conflict and an unrelated change
+	cat <<-EOF >tmp &&
+	diff --git a/numbers b/numbers
+	remerge CONFLICT (content): Merge conflict in numbers
+	index a1fb731..6875544 100644
+	--- a/numbers
+	+++ b/numbers
+	@@ -1,13 +1,9 @@
+	 1
+	 2
+	-<<<<<<< b0ed5cb (change_a)
+	-three
+	-=======
+	-tres
+	->>>>>>> 6cd3f82 (change_b)
+	+drei
+	 4
+	 5
+	 6
+	 7
+	-eight
+	+acht
+	 9
+	EOF
+	sed -e "s/[0-9a-f]\{7,\}/HASH/g" tmp >expect &&
+	git diff-tree -r -p --remerge-diff --no-commit-id ab_resolution >tmp &&
+	sed -e "s/[0-9a-f]\{7,\}/HASH/g" tmp >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'setup non-content conflicts' '
 	git switch --orphan base &&