icase pathspec magic support in ls-tree

Hi folks,

I just found out today both that icase magic exists (awesome!), and that it isn't supported in ls-tree (boo).

As far as I can tell, getting it supported would be the only way to *efficiently* prevent caseless-duplicate files from being created in a repo, in an "update" hook: I'd want to call ls-tree on the new head commit for the branch, passing in an icase pathspec for all the files being added since the previous state - and then sort and uniq.

Of course, for entirely new branches I'd have to do a full check of the tree, and for very large changes that might be the fastest/best thing to do anyway, rather than creating a silly-sized pathspec - but checking the full tree costs me about 1 second, a price that I'm loath to pay for everyday commit verifications of a handful of files, vs a 200,000-file full tree.

I tried changing ls-tree "naively" to just permit the icase magic, without any logic changes, and found at least one case where it doesn't work: when combining wildcards with case-insensitivity, like an icased "T/*" patchspec in the git repo; ls-files finds all the tests, and a naively updated ls-tree does not.

I think I see the last person to update this was Nguyễn Thái Ngọc Duy in 2013, giving a hint as to what would need to be done to make this be supported; is this an area anyone else might be looking at at the moment?

Thanks, Tao