Function write_bundle_header 

pub fn write_bundle_header(
    out: &mut dyn Write,
    algo: HashAlgo,
    prerequisites: &[(ObjectId, Option<String>)],
    refs: &[(ObjectId, String)],
) -> Result<(), BundleError>

Write a bundle header: the signature, the capabilities a non-SHA-1 bundle needs, one -<oid> <subject> line per prerequisite, one <oid> <ref> line per tip, and the blank line the pack follows.

For callers that already have a pack for those tips (such as a push), so a bundle is this header followed by the pack bytes.

Errors

BundleError::Io when writing fails.