Several builtins depend on being able to disable the replace references
so we actually operate on each object individually. These currently do
so by directly mutating the 'read_replace_refs' global.

A future change will move this global into a different place, so it will
be necessary to change all of these lines. However, we can simplify that
transition by abstracting the purpose of these global assignments with a
method call.

We will need to keep this read_replace_refs global forever, as we want
to make sure that we never use replace refs throughout the life of the
process if this method is called. Future changes may present a
repository-scoped version of the variable to represent that repository's
core.useReplaceRefs config value, but a zero-valued read_replace_refs
will always override such a setting.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>

The 'read_replace_objects' constant is initialized by git_default_config
(if core.useReplaceRefs is disabled) and within setup_git_env (if
GIT_NO_REPLACE_OBJECTS) is set. To ensure that this variable cannot be
set accidentally in other places, wrap it in a replace_refs_enabled()
method.

Since we still assign this global in config.c, we are not able to remove
the global scope of this variable and make it a static within
replace-object.c. This will happen in a later change which will also
prevent the variable from being read before it is initialized.

Centralizing read access to the variable is an important first step.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>

The 'read_replace_refs' global specifies whether or not we should
respect the references of the form 'refs/replace/<oid>' to replace which
object we look up when asking for '<oid>'. This global has caused issues
when it is not initialized properly, such as in b6551fe (merge-tree:
load default git config, 2023-05-10).

To make this more robust, move its config-based initialization out of
git_default_config and into prepare_repo_settings(). This provides a
repository-scoped version of the 'read_replace_refs' global.

The global still has its purpose: it is disabled process-wide by the
GIT_NO_REPLACE_OBJECTS environment variable or by a call to
disable_replace_refs() in some specific Git commands.

Since we already encapsulated the use of the constant inside
replace_refs_enabled(), we can perform the initialization inside that
method, if necessary. This solves the problem of forgetting to check the
config, as we will check it before returning this value.

Due to this encapsulation, the global can move to be static within
replace-object.c.

There is an interesting behavior change possible here: we now have a
repository-scoped understanding of this config value. Thus, if there was
a command that recurses into submodules and might follow replace refs,
then it would now respect the core.useReplaceRefs config value in each
repository.

'git grep --recurse-submodules' is such a command that recurses into
submodules in-process. We can demonstrate the granularity of this config
value via a test in t7814.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>