"git config --unset a.b", when "a.b" is the last variable in an
otherwise empty section "a", left an empty section "a" behind, and
worse yet, a subsequent "git config a.c value" did not reuse that
empty shell and instead created a new one.  These have been
(partially) corrected.

* js/empty-config-section-fix:
  git_config_set: reuse empty sections
  git config --unset: remove empty sections (in the common case)
  git_config_set: make use of the config parser's event stream
  git_config_set: do not use a state machine
  config_set_store: rename some fields for consistency
  config: avoid using the global variable `store`
  config: introduce an optional event stream while parsing
  t1300: `--unset-all` can leave an empty section behind (bug)
  t1300: add a few more hairy examples of sections becoming empty
  t1300: remove unreasonable expectation from TODO
  t1300: avoid relying on a bug
  config --replace-all: avoid extra line breaks
  t1300: demonstrate that --replace-all can "invent" newlines
  t1300: rename it to reflect that `repo-config` was deprecated
  git_config_set: fix off-by-two