config: learn `git_protected_config()`

`uploadpack.packObjectsHook` is the only 'protected configuration only'
variable today, but we've noted that `safe.directory` and the upcoming
`safe.bareRepository` should also be 'protected configuration only'. So,
for consistency, we'd like to have a single implementation for protected
configuration.

The primary constraints are:

1. Reading from protected configuration should be fast. Nearly all "git"
   commands inside a bare repository will read both `safe.directory` and
   `safe.bareRepository`, so we cannot afford to be slow.

2. Protected configuration must be readable when the gitdir is not
   known. `safe.directory` and `safe.bareRepository` both affect
   repository discovery and the gitdir is not known at that point [1].

The chosen implementation in this commit is to read protected
configuration and cache the values in a global configset. This is
similar to the caching behavior we get with the_repository->config.

Introduce git_protected_config(), which reads protected configuration
and caches them in the global configset protected_config. Then, refactor
`uploadpack.packObjectsHook` to use git_protected_config().

The protected configuration functions are named similarly to their
non-protected counterparts, e.g. git_protected_config_check_init() vs
git_config_check_init().

In light of constraint 1, this implementation can still be improved.
git_protected_config() iterates through every variable in
protected_config, which is wasteful, but it makes the conversion simple
because it matches existing patterns. We will likely implement constant
time lookup functions for protected configuration in a future series
(such functions already exist for non-protected configuration, i.e.
repo_config_get_*()).

An alternative that avoids introducing another configset is to continue
to read all config using git_config(), but only accept values that have
the correct config scope [2]. This technically fulfills constraint 2,
because git_config() simply ignores the local and worktree config when
the gitdir is not known. However, this would read incomplete config into
the_repository->config, which would need to be reset when the gitdir is
known and git_config() needs to read the local and worktree config.
Resetting the_repository->config might be reasonable while we only have
these 'protected configuration only' variables, but it's not clear
whether this extends well to future variables.

[1] In this case, we do have a candidate gitdir though, so with a little
refactoring, it might be possible to provide a gitdir.
[2] This is how `uploadpack.packObjectsHook` was implemented prior to
this commit.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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