git_config_set: reuse empty sections

git_config_set: reuse empty sections

It can happen quite easily that the last setting in a config section is
removed, and to avoid confusion when there are comments in the config
about that section, we keep a lone section header, i.e. an empty
section.

Now that we use the `event_fn` callback, it is easy to add support for
re-using empty sections, so let's do that.

Note: t5512-ls-remote requires that this change is applied *after* the
patch "git config --unset: remove empty sections (in the common case)":
without that patch, there would be empty `transfer` and `uploadpack`
sections ready for reuse, but in the *wrong* order (and sconsequently,
t5512's "overrides work between mixed transfer/upload-pack hideRefs"
would fail).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>