icase pathspec magic support in ls-tree

[Tao Klerks <tao@klerks.biz>]

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