libgit2 tests should not permanently alter system global git config

[tkelman]

Running make test-libgit2 results in the global git config getting overwritten with Test User, Test@User.com. This should be made temporary and local.

edit: apparently it only overrides the global gitconfig if it hasn't been set yet?

[wildart]

It happens only when user.name or user.email is empty. I recall that without these parameters set, libgit2 calls will segfault.

[tkelman]

We should at least detect that situation and put it back to the way it was when we're done.

[vtjnash]

or change $HOME to a temporary location?

[wildart]

Fixed it in #13417