Find the right git info in a project with submodules

[MichaelScofield]

The reason behind this PR:

We have a project "foo" like this:

foo
├── Cargo.toml
├── main.rs
├── my-submodule
│   ├── src
│   │   ├── Cargo.toml
│   │   ├── version.rs
│   │   ├── build.rs

The shadow::new() is called in build.rs, and the shadow_rs::shadow!(build); is in version.rs.

When I build the project "foo", the git info is that of "my-submodule", instead of "foo", which is not what I actually want(want git info of "foo" of course). This PR expose the "build" function to let projects with submodules decide what the right git repo is. PTAL @baoyachi

[baoyachi]

LGTM. Thx @MichaelScofield .

[baoyachi]

@MichaelScofield New version publised, please use:

[dependencies]
...
shadow-rs = "0.31.0"

[build-dependencies]
shadow-rs = "0.31.0"