Symlinks aren't handled correctly

Unsure if this should be here, or in the ghcommit-action repo. It appears likely there's an assumption being made about the file modes when generating the commit blobs. This causing this utility (and dependent action) to generate defunct commits.

Reproducer: Create a new, or modify an existing, symlink to file in the repo. Run the utility (or Github action).

Expected result: the symlink is created/updated to point to the target file.

Actual result: the symlink points to a non-existent file. The name of this file is the contents of the symlink target.

As I'm able to use another action achieve a similar result successfully (qoomon/actions--create-commit), I presume this isn't a limitation of the GH API, but something specific to its usage here.

Start by reproducing the issue with a new or modified symlink, then inspect the utility's commit blob generation and file-mode handling. Compare the generated commit with the expected symlink target behavior and the successful qoomon/actions--create-commit result. Done means the utility and dependent action create or update symlinks that point to the target file rather than treating the target contents as a filename.