builtin/config: introduce "list" subcommand

While git-config(1) has several modes, those modes are not exposed with
subcommands but instead by specifying action flags like `--unset` or
`--list`. This user interface is not really in line with how our more
modern commands work, where it is a lot more customary to say e.g. `git
remote list`. Furthermore, to add to the confusion, git-config(1) also
allows the user to request modes implicitly by just specifying the
correct number of arguments. Thus, `git config foo.bar` will retrieve
the value of "foo.bar" while `git config foo.bar baz` will set it to
"baz".

Overall, this makes for a confusing interface that could really use a
makeover. It hurts discoverability of what you can do with git-config(1)
and is comparatively easy to get wrong. Converting the command to have
subcommands instead would go a long way to help address these issues.

One concern in this context is backwards compatibility. Luckily, we can
introduce subcommands without breaking backwards compatibility at all.
This is because all the implicit modes of git-config(1) require that the
first argument is a properly formatted config key. And as config keys
_must_ have a dot in their name, any value without a dot would have been
discarded by git-config(1) previous to this change. Thus, given that
none of the subcommands do have a dot, they are unambiguous.

Introduce the first such new subcommand, which is "git config list". To
retain backwards compatibility we only conditionally use subcommands and
will fall back to the old syntax in case no subcommand was detected.
This should help to transition to the new-style syntax until we
eventually deprecate and remove the old-style syntax.

Note that the way we handle this we're duplicating some functionality
across old and new syntax. While this isn't pretty, it helps us to
ensure that there really is no change in behaviour for the old syntax.

Amend tests such that we run them both with old and new style syntax.
As tests are now run twice, state from the first run may be still be
around in the second run and thus cause tests to fail. Add cleanup logic
as required to fix such tests.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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