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>