tag, branch, for-each-ref: add --ignore-case for sorting and filtering

tag, branch, for-each-ref: add --ignore-case for sorting and filtering

This options makes sorting ignore case, which is great when you have
branches named bug-12-do-something, Bug-12-do-some-more and
BUG-12-do-what and want to group them together. Sorting externally may
not be an option because we lose coloring and column layout from
git-branch and git-tag.

The same could be said for filtering, but it's probably less important
because you can always go with the ugly pattern [bB][uU][gG]-* if you're
desperate.

You can't have case-sensitive filtering and case-insensitive sorting (or
the other way around) with this though. For branch and tag, that should
be no problem. for-each-ref, as a plumbing, might want finer control.
But we can always add --{filter,sort}-ignore-case when there is a need
for it.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>