Code clean-up.

* ma/parse-maybe-bool:
  parse_decoration_style: drop unused argument `var`
  treewide: deprecate git_config_maybe_bool, use git_parse_maybe_bool
  config: make git_{config,parse}_maybe_bool equivalent
  config: introduce git_parse_maybe_bool_text
  t5334: document that git push --signed=1 does not work
  Doc/git-{push,send-pack}: correct --sign= to --signed=