Allow overriding of global and system configuration

[sisp]

Git v2.32.0 introduced two new environment variables, GIT_CONFIG_GLOBAL and GIT_CONFIG_SYSTEM, for overriding global and system configurations which is especially useful for executing git commands in a sanitized environment. Since Dulwich already supports GIT_CONFIG_NOSYSTEM, I think it would be great to also add these two environment variables.

See git/git@4179b48 for the original commit in the git project.

[jelmer]

That makes sense; not sure if I'll have time to look at this myself anytime soon, but happy to review PRs.