Add useHttpPath support in CredentialHelper

[arlosi]

Respect the useHttpPath git configuration parameter when calling the git credential helper.

[alexcrichton]

Looks good to me, thanks!

[sharkdp]

This changeset uses str::strip_prefix which is only available in Rust 1.45. The git2 crate previously built fine with 1.42 (and possibly older versions).

I realize that you don't have a strict policy on this ("git2-rs works with stable Rust, and typically works with the most recent prior stable release as well"), but I thought it would be worth mentioning. Bumping the min. supported Rust version in a patch release like this breaks downstream builds (with Rust < 1.45) that simply use

git2 = "0.13"

The README also says "Check the MSRV job of the CI script to see the oldest version of Rust known to pass tests.", but I don't see an explicit requirement in https://github.com/rust-lang/git2-rs/blob/455098a8a1ce24305faf690264fc5b636d210739/.github/workflows/main.yml

Also, thank you for creating and maintaining this library - it has been a pleasure to work with!