More fixes for the "commit-body" (#26898)

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.

templates/repo/commit_page.tmpl

@@ -135,7 +135,7 @@
 				{{end}}
 			</div>
 			{{if IsMultilineCommitMessage .Commit.Message}}
-				<pre class="commit-body gt-mt-0">{{RenderCommitBody $.Context .Commit.Message $.RepoLink $.Repository.ComposeMetas}}</pre>
+				<pre class="commit-body">{{RenderCommitBody $.Context .Commit.Message $.RepoLink $.Repository.ComposeMetas}}</pre>
 			{{end}}
 			{{template "repo/commit_load_branches_and_tags" .}}
 		</div>