catfile: Backport patches to fix leaking catfile processes (v15.0)

Backport patches that fix leaking catfile processes in Gitaly. This backports the following three MRs:

• !4581 (merged) to fix a potential deadlock in the gitpipe package.
• !4590 (merged) to fix a locking issue in the catfile package.
• !4598 (merged) to increase test coverage.