builtin/config: special-case retrieving colors without a key

Our documentation for git-config(1) has a section where it explains how
to parse and use colors as Git would configure them. In order to get the
ANSI color escape sequence to reset the colors to normal we recommend
the following command:

$ git config get --type=color --default="reset" ""

This command is not supposed to parse any configuration keys. Instead,
it is expected to parse the "reset" default value and turn it into a
proper ANSI color escape sequence.

It was reported though [1] that this command doesn't work:

$ git config get --type=color --default="reset" ""
error: key does not contain a section:

This error was introduced in 4e51389000 (builtin/config: introduce "get"
subcommand, 2024-05-06), where we introduced the "get" subcommand to
retrieve configuration values. The preimage of that commit used `git
config --get-color "" "reset"` instead, which still works.

This use case is really quite specific to parsing colors, as it wouldn't
make sense to give git-config(1) a default value and an empty config key
only to return that default value unmodified. But with `--type=color` we
don't return the value directly; we instead parse the value into an ANSI
escape sequence.

As such, we can easily special-case this one use case:

- If the provided config key is empty;

- the user is asking for a color code; and

- the user has provided a default value,

then we call `get_color()` directly. Do so to make the documented
command work as expected.

[1]: <aI+oQvQgnNtC6DVw@szeder.dev>

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>