Divergent hash length behaviour from git when running `git describe --tags --always`

[jpcorreia99]

Hey there!

Noticed that when using git2 to compute the result of git describe, the behaviour can diverge from git

In particular this code

pub fn git_describe_native(path: &Path) -> Result<String> {
    let repo = Repository::open(path)?;

    let mut describe_opts = DescribeOptions::new();
    describe_opts.describe_tags();
    describe_opts.show_commit_oid_as_fallback(true);

    let description = repo.describe(&describe_opts).into_diagnostic()?;

    let format_opts = DescribeFormatOptions::new();
    description.format(Some(&format_opts)).into_diagnostic()?;
}

will differ from git describe --tags --always in specific cases where the repo grows too large.

According to git docs the length of the abbreviation scales as the repository grows, using the approximate number of objects in the repository and a bit of math around the birthday paradox, and defaults to a minimum of 7.

[ehuss]

This would likely be a better issue for https://github.com/libgit2/libgit2/issues, since this is just a thin wrapper around that library.

Additionally, it would probably be best to include more detail such as exactly how it diverges, what aspects of the repository are important, what the different values are, and what you expect.