Slow git add . performance in large repo

[Yissachar Radcliffe <yissachar@block.xyz>]

We have a relatively large git repo and have noticed that `git add .`
operations are slow (~1.5-2s). We have core.fsmonitor and
core.untrackedCache set to true and `git status` executes in ~300ms.
When I turn on trace2 I can see that almost all the time is spent in
read_directo and it's visiting 26960 directories and 77989 paths.

I can use `git add <foo>` or `git add -u .` to speed things up but
`git add .` is the most convenient for us. I created a small script to
pipe the results of `git status` to `git add` and that runs in <500ms.
This leaves me confused as to why the built-in performance is so slow.

git version is 2.49.0, mac os x 15.3