This lowers memory consumption by writing packfiles above a given
threshold to disk. This reduces the memory pressure on workers (but increases
the disk churn), and also allows to use the git loader on more, memory constrained, systems.

As there is a single temporary file which we hold open, we can use the default
Python tempfile feature which unlinks the temporary file directly, allowing the
file to be reaped as soon as the process disappears, even if the process gets
killed. This avoids the need for any manual tempfile cleanup.

Related to T3025

This has been exercised on large repositories (e.g. linux.git yields
a packfile that is almost 4GiB).