changelog: add missing link

[tormath1]

Everything is in the title.

[krnowak]

Won't this reappear in the changelogs of the next release because of the changed timestamp?

[chewi]

I thought it was done by git history, but the show-changes script is hard to understand. Point still stands though. Sorry for missing the link, but I think adding it now might just be more confusing.

[pothos]

Looked the part up again (the end of the script does the actual work):

      git -C "${NEWREPOPATH}" difftool --no-prompt --extcmd='sh -c "cat \"$REMOTE\"" --' "${OLDREF}..${NEWREF}" -- "${NEWPREPEND}changelog/${section}/" | sort || { echo "Error: git difftool failed" ; exit 1 ; }
    fi
    # The -x 'sh -c "cat \"$REMOTE\"" --' command assumes that new changes have their own changelog files,
    # and thus ignores the LOCAL file (which is the empty /dev/null) and prints out the REMOTE completly.
    # If an existing file got changed, we assume that this is just a correction for the old change but
    # shouldn't be included in the release notes again.

Reading my comment there I think it's not fully true when the file exists in both and thus LOCAL and REMOTE exist because cat will still print out the contents of the file again and not ignore it. Maybe we could make it a bit more clever with if [ \"$LOCAL\" = /dev/null ]; then cat … ; fi so that it only prints the changelog entry if it's a new one.

[pothos]

Opened a PR with a fix:
flatcar/flatcar-build-scripts#178