Cargo: fix git credential helper issue

[hmarr]

Cargo (well, git2-rs) doesn't accept the 'command with args' form of specifying a git credential helper. It does, however, accept the shell snippet form, which is prefixed with a !.

I still need to confirm that this doesn't break regular git (all other languages).

[hmarr]

For future reference:

• Here's the PR to Cargo that prevents it hanging: Don't retry invalid credentials from git credential helpers rust-lang/cargo#6681
• And here's the git2-rs PR that fixes the original issue with how Git credential helpers are specified: Don't quote absolute path helper commands rust-lang/git2-rs#402