Use directory-specific GitCommit in stackbrew library output

Instead of using the branch head commit for all images, find the last
commit that actually changed each image directory. This ensures the
GitCommit field accurately reflects when each image was last modified.

Author: cobexer

generate-stackbrew-library.py

@@ -73,6 +73,11 @@ def sort_key(d):
     dirs.sort(key=sort_key)
     return dirs
 
+def get_directory_commit(branch_commit, dir_path):
+    """Get the last commit that changed the given directory."""
+    output = run_command(["git", "log", "-1", "--format=%H", branch_commit, "--", f"{dir_path}/"])
+    return output.strip()
+
 def get_arches(image, cache):
     if image in cache:
         return cache[image]
@@ -145,7 +150,8 @@ def main():
 
         first_version = None
         for dir_path in directories:
-            dockerfile = run_command(["git", "show", f"{commit}:{dir_path}/Dockerfile"])
+            dir_commit = get_directory_commit(commit, dir_path)
+            dockerfile = run_command(["git", "show", f"{dir_commit}:{dir_path}/Dockerfile"])
 
             # Extract FROM
             from_match = re.search(r"^\s*FROM\s+(\S+)", dockerfile, re.MULTILINE | re.IGNORECASE)
@@ -298,7 +304,7 @@ def main():
             print(f"\nTags: {', '.join(actual_tags)}")
             print(f"Architectures: {arches}")
             print(f"GitFetch: refs/heads/{branch}")
-            print(f"GitCommit: {commit}")
+            print(f"GitCommit: {dir_commit}")
             print(f"Directory: {dir_path}")
 
 if __name__ == "__main__":