Add commit list header

Description

This is part of a bigger effort to refactor the repository Commit list.

As part of this issue we should implement the header for the commits page.

🎨 Figma

Desktop

	Screen	Notes
Default		Breadcrumb behavior remains the same as in the old Commits page. Breadcrumbs link to the Commits page for the respective file/directory. We should keep an eye on user feedback about whether or not users want more density on the Commits page / want to see more commits per page.
Non-default branch		The View open merge request button in the old Commits page will be replaced by the open MRs badge implemented in #520125 (closed). [out of scope for this specific issue] In this issue: keep the old button, for now.
Menu		The Browse files and Commits feed button will be moved into the overflow menu.

Mobile

	Screen	Notes
Default		Breadcrumbs appear by default to the right of the branch selector
Breadcrumb overflow		If the breadcrumbs grow larger than the available space, the breadcrumbs will move underneath the branch selector. See mobile Repository page for an example of this behavior.