environment: move trust_executable_bit into repo_config_values

environment: move trust_executable_bit into repo_config_values

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

To ensure code readability, the getter function
'repo_trust_executable_bit()' has been introduced.
Callers access this configuration by passing in 'repo'
when possible, and explicitly fall back to 'the_repository'
the rest of time.

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>