Stop using `the_repository` in `pack-write.c`

This is a small series to remove global variable usage from pack-write.c. Mostly it bubble's up the usage of global variables to upper layers. The only exception is in write-midx.c, which was cleaned of global variable usage, so there, we use the repo that is in available in the context.

This series is based on fbe8d307 (Git 2.48, 2025-01-10) with 'ps/the-repository' merged in.

Closes: #458 (closed)