[proposal] make parse-options nicer wrt optional arguments (supersedes all my recent posts on the matter)

Here is a series that aims at fixing the various issues with parse-options that were raised recently.

* preliminary patch:
  [PATCH 1/7] parse-options: Make callbacks take flags instead of boolean `unset`

* teach git parse-options to allow callbacks to ignore arguments that
  don't seem to be theirs, refactors:
  [PATCH 2/7] parse-options: allow callbacks to ignore arguments they don't need to use.
  [PATCH 3/7] parse-options: Let the integer/string cases be callbacks as well.
  [PATCH 4/7] parse-options: let OPT__ABBREV ignore arguments.

* Document this (my previous proposal + Junio's squashed):
  [PATCH 5/7] parse-options: Add a gitcli(5) man page.

* Implement my `{}` proposal, a sed -e s/{}/_/ will replace {} with _
  as a wildcard. Contains documentation for this placeholder.
  [PATCH 6/7] parse-options: have a `use default value` wildcard.

* Somehow unrelated patch, but still parse-option related (resend):
  [PATCH 7/7] git-tag: fix -l switch handling regression.

This has been pushed as my ph/parseopt branch on git://git.madism.org/git.git.