completion: add and use __git_compute_second_level_config_vars_for_section

completion: add and use __git_compute_second_level_config_vars_for_section

In a previous commit we removed some hardcoded config variable names from
function __git_complete_config_variable_name in the completion script by
introducing a new function,
__git_compute_first_level_config_vars_for_section.

The remaining hardcoded config variables are "second level"
configuration variables, meaning 'branch.<name>.upstream',
'remote.<name>.url', etc. where <name> is a user-defined name.

Making use of the new existing --config flag to 'git help', add a new
function, __git_compute_second_level_config_vars_for_section. This
function takes as argument a config section name and computes the
corresponding second-level config variables, i.e. those that contain a
'<' which indicates the start of a placeholder. Note that as in
__git_compute_first_level_config_vars_for_section added previsouly, we
use indirect expansion instead of associative arrays to stay compatible
with Bash 3 on which macOS is stuck for licensing reasons.

As explained in the previous commit, we use the existing pattern in the
completion script of using global variables to cache the list of
variables for each section.

Use this new function and the variables it defines in
__git_complete_config_variable_name to remove hardcoded config
variables, and add a test to verify the new function.  Use a single
'case' for all sections with second-level variables names, since the
code for each of them is now exactly the same.

Adjust the name of a test added in a previous commit to reflect that it
now tests the added function.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>