Allow the release info build script to work without requiring Git to be installed

[timon-schelling]

• Prevents panic that happens when graphite was build in an environment without git. (len("unkonwn") < 8)
• Adds option to set release information environment variables, useful for build environments where git is not available.
• Tell cargo when the build script needs to be run again.
• Improve readability of build script.

[Keavon]

Why --abbrev-ref?

[timon-schelling]

Returns a commit hash otherwise. But I could also use name-rev like you did before.

[Keavon]

What does it return if not a commit hash?

[timon-schelling]

Branch name. (Rev short name if you want to use git wording)

[Keavon]

We should aim to always have a commit hash so it's possible to actually look up which commit a build link belongs to. (This is also something we don't do on https://editor.graphite.rs but should do.)

[timon-schelling]

Commit hash is collected in the statement above. There rev-parse without --abbrev-ref.

[Keavon]

What is the purpose of this second one?

[timon-schelling]

// Collect commit hash
        let commit_hash = env_or_else("GRAPHITE_GIT_COMMIT_HASH", || git_or_unknown(&["rev-parse", "HEAD"]));
// Collect Branch name
         let commit_branch = env_or_else("GRAPHITE_GIT_COMMIT_BRANCH", || { 
                 let gh = env::var("GITHUB_HEAD_REF").unwrap_or_default();

// Code you comment on is used to collect the branch name when it is not set by GitHub locally for now example

[Keavon]

Oh, I wasn't looking closely enough. Disregard my last two comments. Anyway, I think we're good to merge.