[RFC/PATCH 0/9] port tag.c to use ref-filter library

This is part of the GSoC project to unify the commands `git tag -l`, `git branch -l` and `git for-each-ref`.

This is a follow up to http://article.gmane.org/gmane.comp.version-control.git/272641 which is still in the 6th iteration.

This ports over tag.c to use ref-filter APIs and adds --format, --sort, --merged and --no-merged options to the same.

 Documentation/git-tag.txt |  39 ++++++++++---
 builtin/for-each-ref.c    |   3 +-
 builtin/tag.c             | 362
++++++++++++++++++++------------------------------------------------------------------------------------------------
 ref-filter.c              | 100 ++++++++++++++++++++++++++++----
 ref-filter.h              |   7 ++-
 t/t7004-tag.sh            |  51 ++++++++++++++---
 6 files changed, 233 insertions(+), 329 deletions(-)