environment: move pager_program into repo_config_values

environment: move pager_program into repo_config_values

The 'pager_program' variable is currently defined as a file-scoped
static string in pager.c. Move it into 'struct repo_config_values'.

The configuration parsing logic remains strictly within pager.c to
respect subsystem boundaries. The read/write operations are simply
redirected to the repository-specific structure using
'repo_config_values()'. All current callers indeed pass
'the_repository', so this new enforcement does not harm them.

Similar to the recent editor_program migration, no standalone getter
is introduced to keep the code minimal. The dynamically allocated
memory is now managed by 'repo_config_values_clear()'.

On top of that, fix memory leaks in pager.c while we are at it.

Mentored-by: Christian Couder <redacted>
Mentored-by: Ayush Chandekar <redacted>
Mentored-by: Olamide Caleb Bello <redacted>
Signed-off-by: Tian Yuchen <redacted>
Signed-off-by: Junio C Hamano <redacted>