For git SHA and dirty, use .cargo_vcs_info.json if in a published crate

[hgomersall]

Apparently using a published crate populates VERGEN_GIT_SHA and VERGEN_GIT_DIRTY with the default (idempotent) output.

Shameless stealing from https://github.com/fedimint/fedimint/pull/3735/files, the following example in my build.rs might be a better option (perhaps enabled optionally):

use std::error::Error;
use vergen_git2::{Emitter, Git2Builder};

fn main() -> Result<(), Box<dyn Error>> {

    if let Ok(file) = std::fs::File::open("./.cargo_vcs_info.json") {
        let info: serde_json::Value = serde_json::from_reader(file)
            .map_err(|e| format!("Failed to parse .cargo_vcs_info.json: {}", e))?;
        let hash = info["git"]["sha1"].as_str().ok_or_else(|| {
            format!(
                "Failed to parse .cargo_vcs_info.json: no `.git.sha` field: {:?}",
                info
            )
        })?;
        let dirty = info["git"]["dirty"].as_bool().ok_or_else(|| {
            format!(
                "Failed to parse .cargo_vcs_info.json: no `.git.dirty` field: {:?}",
                info
            )
        })?;
        println!("cargo:rustc-env=VERGEN_GIT_DIRTY={dirty}");
        println!("cargo:rustc-env=VERGEN_GIT_SHA={hash}");
        return Ok(());
    } else {
        let gitcl = Git2Builder::default()
            .dirty(false)
            .sha(false)
            .build()?;

        Emitter::default()
            .add_instructions(&gitcl)?
            .emit()?;
    }
    Ok(())
}

This uses .cargo_vcs_info.json which is provided in the crate.

Interestingly, Gemini is convinced Vergen already supports this, though my investigation fell short, hence this issue.

[hgomersall]

It seems that the dirty flag only gets populated when it is actually dirty (and the --allow-dirty flag is set). This means the error should probably be a fallback to dirty = false.