Use git clone --no-checkout and git checkout --force

## What does this MR do?

Adds --no-checkout to `git clone` and -f to `git checkout`

## Why was this MR needed?

On a case-insensitive filesystem, mixed-case commits can unconditionally break both clone and fetch, as the initial, implicit checkout of `HEAD` in `git clone`immediately modifies the working tree, which causes the later `git checkout` to fail (without -f). `--no-checkout` skips the implicit checkout, and should be fine in all circumstances.

With the fetch strategy, this still isn't enough as the case-insensitivity-caused change persists, and the checkout for the second build fails. `-f` forces it to proceed, "overwriting" the local changes. This also prevents local modifications brought about by CR/LF differences, or others not caught by the `git clean` and `git reset`, from breaking the fetch strategy.

Both these arguments are present in git v1.6.1

## Are there points in the code the reviewer needs to double check?

Are there circumstances where we want the build to fail, that `git checkout -f` would in fact succeed in? I can't think of any offhand.

## Does this MR meet the acceptance criteria?

- [ ] Documentation created/updated
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)

## What are the relevant issue numbers?

Closes #1296

See merge request !341