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

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
  363337e6eb812d0c0d785ed4261544f35559ff8b
  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>