chore(scm): gate manual file hash behind git2 (#10088)
### Description
We're getting some dead code warning when building `@turbo/repository`:
```
warning: function `git_like_hash_file` is never used
Warning: --> crates/turborepo-scm/src/manual.rs:12:4
|
12 | fn git_like_hash_file(path: &AbsoluteSystemPath) -> Result<String, Error> {
| ^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(dead_code)]` on by default
warning: function `to_glob` is never used
Warning: --> crates/turborepo-scm/src/manual.rs:31:4
|
31 | fn to_glob(input: &str) -> Result<Glob, Error> {
| ^^^^^^^
warning: function `hash_files` is never used
Warning: --> crates/turborepo-scm/src/manual.rs:38:15
|
38 | pub(crate) fn hash_files(
| ^^^^^^^^^^
warning: function `get_package_file_hashes_without_git` is never used
Warning: --> crates/turborepo-scm/src/manual.rs:59:15
|
59 | pub(crate) fn get_package_file_hashes_without_git<S: AsRef<str>>(
```
Gate this module behind `git2` to silence it.
### Testing Instructions
`cargo build -p turborepo-napi` should not produce any errors
