merge-ort: initialize repo in index state

merge-ort: initialize repo in index state

initialize_attr_index() does not initialize the repo member of
attr_index. Starting in 44451a2e5e (attr: teach "--attr-source=<tree>"
global option to "git", 2023-05-06), this became a problem because
istate->repo gets passed down the call chain starting in
git_check_attr(). This gets passed all the way down to
replace_refs_enabled(), which segfaults when accessing r->gitdir.

Fix this by initializing the repository in the index state.

Signed-off-by: John Cai <redacted>
Helped-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>