environment: move editor_program into repo_config_values
The global variable 'editor_program' holds the path to the user's preferred editor. Move 'editor_program' into 'struct repo_config_values' to continue the libification effort. There have been discussions on whether external programs like editors truly need to be configured on a per-repository basis within the same process. While a single process might rarely invoke different editors, this migration is necessary for two reasons: 1. Developers frequently use different toolchains for different projects. Per-repo configuration respects this. 2. Moving this string into 'repo_config_values' eliminates mutable global state. As the codebase moves toward becoming a long-running processes, managing multiple repositories concurrently must not overwrite each other's program configurations. No standalone getter function is introduced. Callers directly access the field via 'repo_config_values()'. Heap memory is safely reclaimed in 'repo_config_values_clear()'. 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>