Initially, the setting has been solely used to enable the use of
`fsync()` when creating objects. Since then, the use has been extended
to also cover references and index files. As the option is not yet part
of any release, we can still correct this by renaming the option to
something more sensible, indicating not only correlation to objects.

This commit renames the option to `GIT_OPT_ENABLE_FSYNC_GITDIR`. We also
move the variable from the object to repository source code.

Rename `GIT_OPT_ENABLE_SYNCHRONIZED_OBJECT_CREATION` ->
`GIT_OPT_ENABLE_SYNCHRONOUS_OBJECT_CREATION`.

Allow users to enable `SYNCHRONIZED_OBJECT_CREATION` with a setting.

This allows lighter weight validation in `git_object__is_valid` that
does not require reading the entire object.

When `GIT_OPT_ENABLE_STRICT_OBJECT_CREATION` is turned on, validate
the index entries given to `git_index_add`.

Removed useless prototype and renamed object typecast functions
declaration macro.

This removes the GIT_INLINE versions of the simple git_object
accessors and standardizes them with a helper macro in src/object.h
to build the function bodies.