ref-filter: apply --ignore-case to all sorting keys

ref-filter: apply --ignore-case to all sorting keys

All of the ref-filter users (for-each-ref, branch, and tag) take an
--ignore-case option which makes filtering and sorting case-insensitive.
However, this option was applied only to the first element of the
ref_sorting list. So:

  git for-each-ref --ignore-case --sort=refname

would do what you expect, but:

  git for-each-ref --ignore-case --sort=refname --sort=taggername

would sort the primary key (taggername) case-insensitively, but sort the
refname case-sensitively. We have two options here:

  - teach callers to set ignore_case on the whole list

  - replace the ref_sorting list with a struct that contains both the
    list of sorting keys, as well as options that apply to _all_
    keys

I went with the first one here, as it gives more flexibility if we later
want to let the users set the flag per-key (presumably through some
special syntax when defining the key; for now it's all or nothing
through --ignore-case).

The new test covers this by sorting on both tagger and subject
case-insensitively, which should compare "a" and "A" identically, but
still sort them before "b" and "B". We'll break ties by sorting on the
refname to give ourselves a stable output (this is actually supposed to
be done automatically, but there's another bug which will be fixed in
the next commit).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>