config.c: don't assign to "cf_global" directly

To make "cf_global" easier to remove, replace all direct assignments to
it with function calls. This refactor has an additional maintainability
benefit: all of these functions were manually implementing stack
pop/push semantics on "struct config_source", so replacing them with
function calls allows us to only implement this logic once.

In this process, perform some now-obvious clean ups:

- Drop some unnecessary "cf_global" assignments in
  populate_remote_urls(). Since it was introduced in 399b198489 (config:
  include file if remote URL matches a glob, 2022-01-18), it has stored
  and restored the value of "cf_global" to ensure that it doesn't get
  accidentally mutated. However, this was never necessary since
  "do_config_from()" already pushes/pops "cf_global" further down the
  call chain.

- Zero out every "struct config_source" with a dedicated initializer.
  This matters because the "struct config_source" is assigned to
  "cf_global" and we later 'pop the stack' by assigning "cf_global =
  cf_global->prev", but "cf_global->prev" could be pointing to
  uninitialized garbage.

  Fortunately, this has never bothered us since we never try to read
  "cf_global" except while iterating through config, in which case,
  "cf_global" is either set to a sensible value (when parsing a file),
  or it is ignored (when iterating a configset). Later in the series,
  zero-ing out memory will also let us enforce the constraint that
  "cf_global" and "current_config_kvi" are never non-NULL together.

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

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