environment: move has_symlinks into repo_config_values

environment: move has_symlinks into repo_config_values

Move the global 'has_symlinks' configuration into the
repository-specific 'repo_config_values' struct.

Introduce 'repo_has_symlinks()' getter for readability.
Callers access this configuration by passing in 'repo'
when possible, and explicitly fall back to
'the_repository' the rest of the time.

Introduce 'platform_has_symlinks()' macro to allow
platform specific-customization, primarily to help MinGW.
Platforms can override this in their respective headers.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Ayush Chandekar <ayu.chandekar@gmail.com>
Mentored-by: Olamide Caleb Bello <belkid98@gmail.com>
Signed-off-by: Tian Yuchen <cat@malon.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>