The changes for "commit-body" in #26877 are not ideal.
The reason is: the "commit-body" is usually a `<pre>`, it has default
margins. In most cases, we do not need that large margin. So, this PR
introduces a general but small margin for all "commit-body" elements.
Then these `gt-m-0` could be removed.
The `:not` selector is not needed, because the `.timeline-item` selector
is already clear enough.
682552378f