Fast-track of 'git config list --type' correctness (#863)

See gitgitgadget#2044 for the original review of this change.

The main feature here in Git is that `git config list --type=<X>` works
correctly.

But the real benefit is that we can update GCM to use `git config list
--type=<X>` to get the full list of config and store those values in the
cache without dozens of `git config --get` calls.

See git-ecosystem/git-credential-manager#2268 for the change that adds
this cache with a version check for Git 2.54.0. If we additionally add
derrickstolee/Git-Credential-Manager-Core#1, then a v2.53.0.vfs.0.1
release would also allow this behavior change.

I tested all of these features together against my copy of the Office
monorepo running a basic `git fetch`. Note that these runs are not
actually downloading much data, if any.

| Command | Mean [s] | Min [s] | Max [s] | Relative |
|:---|---:|---:|---:|---:|
| Without Cache | 14.986 ± 0.255 | 14.558 | 15.192 | 3.29 ± 0.17 |
| With Cache | 4.561 ± 0.223 | 4.390 | 4.935 | 1.00 |

During this test, I used a single GCM version with these changes and
modified the environment variables to change the Git version executed by
GCM.

* [X] This is an early version of work already under review upstream.
(These exact commits are merged to `next`.)

Author: dscho

Documentation/git-config.adoc

@@ -240,6 +240,9 @@ Valid `<type>`'s include:
   that the given value is canonicalize-able as an ANSI color, but it is written
   as-is.
 +
+If the command is in `list` mode, then the `--type <type>` argument will apply
+to each listed config value. If the value does not successfully parse in that
+format, then it will be omitted from the list.
 
 --bool::
 --int::