RepositoryDiffPage: prevent scroll bar from appearing over content

Created by: pdubroy

Context

Addresses part of #21662 (closed).

The scroll bar on the diff view currently appears over top of the content, rather than over top of the margin between the diff page and the extension sidebar:

The desired behavior is for it to appear slightly further to the right, over top of the margin:

Changes

Change the margin on RepositoryCommitPage to padding.