Release workflow: only commit modified changelogs

[vcanales]

Description

Checks if changelog.txt committed in the context of this workflow is different from the one in trunk and in the release branch. This makes it possible to re-run the workflow in order to re-try the upload of the plugin archive to the directory. Before, since attempting to create a commit without changes exits with an error, the upload step was being skipped (eg. this retried run)

This would've allowed us to use the workflow in order to fix the failure we faced when releasing Gutenberg v11.3.0, where the SVN upload timed out; @youknowriad saved the day committing the plugin manually.

How has this been tested?

Tried these changes in a fork (SVN Upload fails there because I purposefully did not configure secrets.)

1. First run updates the changelog correctly.
2. Subsequent re-runs don't attempt to commit unchanged files.
3. Upload step runs again every time.

[youknowriad]

This is a great improvement. Should we do the same for the SVN commit and tagging as well?

[vcanales]

Should we do the same for the SVN commit and tagging as well?

I'm not proficient enough in svn to tell for sure, but in testing trying to create a commit with no changed files did not result in an error, so the workflow would not fail in this case.