Use directory-specific GitCommit in stackbrew library output

[cobexer]

• Use the last commit that changed each image directory for the GitCommit field, instead of always using the branch head commit
• Adds get_directory_commit() helper that runs git log -1 --format=%H scoped to each directory

[github-actions]

Library file diff (ignoring commit IDs)

134c134,158
< Tags: 9.4.1-jdk-lts-and-current, 9.4-jdk-lts-and-current, 9-jdk-lts-and-current, jdk-lts-and-current, 9.4.1-jdk-lts-and-current-noble, 9.4-jdk-lts-and-current-noble, 9-jdk-lts-and-current-noble, jdk-lts-and-current-noble, 9.4.1-jdk-25-and-25, 9.4-jdk-25-and-25, 9-jdk-25-and-25, jdk-25-and-25, 9.4.1-jdk-25-and-25-noble, 9.4-jdk-25-and-25-noble, 9-jdk-25-and-25-noble, jdk-25-and-25-noble
---
> Tags: 9.4.1-jdk26, 9.4-jdk26, 9-jdk26, jdk26, 9.4.1-jdk26-noble, 9.4-jdk26-noble, 9-jdk26-noble, jdk26-noble
> Architectures: amd64, arm64v8, ppc64le, riscv64, s390x
> GitFetch: refs/heads/master
> GitCommit: 0000000000000000000000000000000000000000
> Directory: jdk26-noble
> 
> Tags: 9.4.1-jdk26-alpine, 9.4-jdk26-alpine, 9-jdk26-alpine, jdk26-alpine
> Architectures: amd64, arm64v8
> GitFetch: refs/heads/master
> GitCommit: 0000000000000000000000000000000000000000
> Directory: jdk26-alpine
> 
> Tags: 9.4.1-jdk26-corretto, 9.4-jdk26-corretto, 9-jdk26-corretto, jdk26-corretto, 9.4.1-jdk26-corretto-al2023, 9.4-jdk26-corretto-al2023, 9-jdk26-corretto-al2023, jdk26-corretto-al2023
> Architectures: amd64, arm64v8
> GitFetch: refs/heads/master
> GitCommit: 0000000000000000000000000000000000000000
> Directory: jdk26-corretto
> 
> Tags: 9.4.1-jdk26-ubi, 9.4-jdk26-ubi, 9-jdk26-ubi, jdk26-ubi, 9.4.1-jdk26-ubi10, 9.4-jdk26-ubi10, 9-jdk26-ubi10, jdk26-ubi10
> Architectures: amd64, arm64v8, ppc64le, s390x
> GitFetch: refs/heads/master
> GitCommit: 0000000000000000000000000000000000000000
> Directory: jdk26-ubi10
> 
> Tags: 9.4.1-jdk-lts-and-current, 9.4-jdk-lts-and-current, 9-jdk-lts-and-current, jdk-lts-and-current, 9.4.1-jdk-lts-and-current-noble, 9.4-jdk-lts-and-current-noble, 9-jdk-lts-and-current-noble, jdk-lts-and-current-noble, 9.4.1-jdk-25-and-26, 9.4-jdk-25-and-26, 9-jdk-25-and-26, jdk-25-and-26, 9.4.1-jdk-25-and-26-noble, 9.4-jdk-25-and-26-noble, 9-jdk-25-and-26-noble, jdk-25-and-26-noble
140c164
< Tags: 9.4.1-jdk-lts-and-current-alpine, 9.4-jdk-lts-and-current-alpine, 9-jdk-lts-and-current-alpine, jdk-lts-and-current-alpine, 9.4.1-jdk-25-and-25-alpine, 9.4-jdk-25-and-25-alpine, 9-jdk-25-and-25-alpine, jdk-25-and-25-alpine
---
> Tags: 9.4.1-jdk-lts-and-current-alpine, 9.4-jdk-lts-and-current-alpine, 9-jdk-lts-and-current-alpine, jdk-lts-and-current-alpine, 9.4.1-jdk-25-and-26-alpine, 9.4-jdk-25-and-26-alpine, 9-jdk-25-and-26-alpine, jdk-25-and-26-alpine
146c170
< Tags: 9.4.1-jdk-lts-and-current-corretto, 9.4-jdk-lts-and-current-corretto, 9-jdk-lts-and-current-corretto, jdk-lts-and-current-corretto, 9.4.1-jdk-lts-and-current-corretto-al2023, 9.4-jdk-lts-and-current-corretto-al2023, 9-jdk-lts-and-current-corretto-al2023, jdk-lts-and-current-corretto-al2023, 9.4.1-jdk-25-and-25-corretto, 9.4-jdk-25-and-25-corretto, 9-jdk-25-and-25-corretto, jdk-25-and-25-corretto, 9.4.1-jdk-25-and-25-corretto-al2023, 9.4-jdk-25-and-25-corretto-al2023, 9-jdk-25-and-25-corretto-al2023, jdk-25-and-25-corretto-al2023
---
> Tags: 9.4.1-jdk-lts-and-current-corretto, 9.4-jdk-lts-and-current-corretto, 9-jdk-lts-and-current-corretto, jdk-lts-and-current-corretto, 9.4.1-jdk-lts-and-current-corretto-al2023, 9.4-jdk-lts-and-current-corretto-al2023, 9-jdk-lts-and-current-corretto-al2023, jdk-lts-and-current-corretto-al2023, 9.4.1-jdk-25-and-26-corretto, 9.4-jdk-25-and-26-corretto, 9-jdk-25-and-26-corretto, jdk-25-and-26-corretto, 9.4.1-jdk-25-and-26-corretto-al2023, 9.4-jdk-25-and-26-corretto-al2023, 9-jdk-25-and-26-corretto-al2023, jdk-25-and-26-corretto-al2023