config.c: pass ctx with CLI config

config.c: pass ctx with CLI config

Pass config_context when parsing CLI config. To provide the .kvi member,
refactor out kvi_from_param() from the logic that caches CLI config in
configsets. Now that config_context and config_context.kvi is always
present when config machinery calls config callbacks, plumb "kvi" so
that we can remove all calls of current_config_scope() except for
trace2/*.c (which will be handled in a later commit), and remove all
other current_config_*() (the functions themselves and their calls).
Note that this results in .kvi containing a different, more complete
set of information than the mocked up "struct config_source" in
git_config_from_parameters().

Plumbing "kvi" reveals a few places where we've been doing the wrong
thing:

* git_config_parse_parameter() hasn't been setting config source
  information, so plumb "kvi" there too.

* Several sites in builtin/config.c have been calling current_config_*()
  functions outside of config callbacks (indirectly, via the
  format_config() helper), which means they're reading state that isn't
  set correctly:

  * "git config --get-urlmatch --show-scope" iterates config to collect
    values, but then attempts to display the scope after config
    iteration, causing the "unknown" scope to be shown instead of the
    config file's scope. It's clear that this wasn't intended: we knew
    that "--get-urlmatch" couldn't show config source metadata, which is
    why "--show-origin" was marked incompatible with "--get-urlmatch"
    when it was introduced [1]. It was most likely a mistake that we
    allowed "--show-scope" to sneak through.

    Fix this by copying the "kvi" value in the collection phase so that
    it can be read back later. This means that we can now support "git
    config --get-urlmatch --show-origin", but that is left unchanged
    for now.

  * "git config --default" doesn't have config source metadata when
    displaying the default value, so "--show-scope" also results in
    "unknown", and "--show-origin" results in a BUG(). Fix this by
    treating the default value as if it came from the command line (e.g.
    like we do with "git -c" or "git config --file"), using
    kvi_from_param().

[1] https://lore.kernel.org/git/20160205112001.GA13397@sigill.intra.peff.net/

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>