Merge branch 'jk/for-each-ref-icase'

The "--ignore-case" option of "git for-each-ref" (and its friends)
did not work correctly, which has been fixed.

* jk/for-each-ref-icase:
  ref-filter: avoid backend filtering with --ignore-case
  for-each-ref: consistently pass WM_IGNORECASE flag
  t6300: add a test for --ignore-case

diff --cc ref-filter.c
Simple merge

diff --cc t/t6300-for-each-ref.sh
Simple merge