Fix changelog creation script

[vcanales]

Previous Changelogs have come up empty; this PR ensures that the changes since the last tag created are correctly added to the Changelog.

1. Don't specify --merge, since we're not using merge commits.
2. Prioritize printing the message instead of the body; GitHub automatically sets the squashed commit to the PR title and adds the number. This should always be available, but will still add body as a fallback just in case something is committed without a PR.
3. Use git itself to determine the last tag created, instead of using the package.json file.
4. Other minor fixes — capitalization, order of commits in the output, etc.

Sample output for v2.0.0 from the updated script:

= 2.0.0 =
* Add theme json inspector (#520)
* Add deprecation notice in theme export admin screen (#540)
* Replace font management with screen pointing to native font library (#539)
* Added creation of theme validation to site editor interface (#532)
* Add/child theme creation (#531)
* Add blueprint.json file to enable plugin previews (#511)
* Extracted any logic that may need to be tested from the api class (#522)
* Use CORE for Font Management (#518)
* Add integration tests (#393)
* Remove CODE_OF_CONDUCT.md from .distignore. (#515)
* Remove repo specific CoC. (#514)
* GitHub Actions: Add JavaScript Unit Test (#508)
* Add files and directories not needed for release to `.distignore` (#512)
* Replace dash icon with SVG icon (#506)
* Fix browser warning error when clicking the reset button (#505)
* Add markdown and package.json lint command (#504)
* Fix react warning error on font upload page (#502)
* Fix dynamic property deprecation (#501)
* Add text domain to translation target (#499)

Testing:

Run RELEASE_TYPE=major npm run update-version — Your changelog file will include the commits from this branch, which is fine; when merged, it'll only use the history from trunk.

[mikachan]

Thank you for fixing this! It tests well for me. Running RELEASE_TYPE=major npm run update-version, I see the commits from this branch, formatted as expected:

(The screenshot cuts off the bottom, the last line is "* Add text domain to translation target (#499)".)

I see the screenshot changes are in line with what's described in the plugin docs here, so these look fine too.

I think this is good to bring in 👍

[creativecoder]

This is also working well for me! I tested the update-version command with major and minor versions.

One question, why change screenshots from jpg to png? The png file size is a lot larger.

[vcanales]

One question, why change screenshots from jpg to png? The png file size is a lot larger.

No particular reason, that's just the default for when I took them. I've transformed them to jpeg on ef6add0; thanks for spotting!