git: Allow core.packedGitLimit to be configured

core.packedGitLimit [1] allows us to control the maximum size of packfile data that Git slurps into memory. Tweaking this value was proposed in gitlab-org/charts/gitlab#5795 may alleviate some git-clone(1) performance issues we've seen in large repositories.

Expose an undocumented environment variable which enables us to set arbitrary values for core.packedGitLimit. This knob will be twisted by us, and eventually this option will either go away (because we've found a good way to configure it automatically), or be documented properly as a support-only/experimental option.

[1] https://git-scm.com/docs/git-config#Documentation/git-config.txt-corepackedGitLimit