Replace archived go-git dependency

[Integralist]

Problem: the go-git dependency doesn't support SSH, resulting in strange low-level authentication errors.
Solution: replace go based implementation of git with a subprocess call to the user's git binary.

Example

$ fastly compute init --name test -d "thing" -a "$(whoami)" -f git@github.com:fastly/compute-starter-kit-rust-default.git -l Rust
Creating a new Compute@Edge project.

Press ^C at any time to quit.


✓ Initializing...
✓ Checking if git is installed...
✓ Fetching package template...
✓ Updating package manifest...
✓ Initializing package...

Initialized package test to:
        /Users/integralist/Code/go/testing-cli-cloningrepo

To publish the package (build and deploy), run:
        fastly compute publish

To learn about deploying Compute@Edge projects using third-party orchestration tools, visit:
        https://developer.fastly.com/learning/integrations/orchestration/


SUCCESS: Initialized package test

Now if I tree the current directory...

$ tree
.
├── Cargo.lock
├── Cargo.toml
├── LICENSE
├── README.md
├── SECURITY.md
├── fastly.toml
├── rust-toolchain
└── src
    └── main.rs

If git is not installed you should see:

✗ Initializing...

ERROR: `git` not found in $PATH.

The Fastly CLI requires a local installation of git.  For installation instructions for your operating system see:

        $ https://git-scm.com/book/en/v2/Getting-Started-Installing-Git

[phamann]

LGTM