Add commit list header breadcrumbs

What does this MR do and why?

This is part of a bigger effort to refactor the repository Commit list. All changes are behind the FF project_commits_refactor`. Specifically, this MR adds a file breadcrumb to the commit list header. This matches exactly the existing file breadcrumb when FF is off.

References

Finish state of the header

Implementation Plan

Task	MR
Add ref selector	!200567 (merged)
Add breadcrumb	👈 this MR
Add overflow menu	!200595 (merged)
Add filter search and support author search	!199810 (merged)
Add support for message search	!200595 (merged)

Screenshots or screen recordings

⚠️ the list of commits uses mock data and would not change when file path gets updated. to validate this change, i confirm the link generated from the new breadcrumb matches the existing one (when FF is off) 👍

FF off	before change, FF on	after change, FF on

project root https://gdk.test:3443/reported_user_22/gitlab-shell/-/commits/main	CleanShot 2025-08-11 at 15.43.12.mp4
file on a non-default branch https://gdk.test:3443/reported_user_22/gitlab-shell/-/commits/ashmckenzie/debug-yamux-issues/doc/beginners_guide.md	CleanShot 2025-08-11 at 15.41.48.mp4
file with a tag https://gdk.test:3443/reported_user_22/gitlab-shell/-/commits/v14.38.0/spec/gitlab_shell_authorized_keys_check_spec.rb	CleanShot 2025-08-11 at 15.40.49.mp4

How to set up and validate locally

To test commits page for a blob (file):

• Go to a file inside a repository
• Click History in the last commit block

To test commits page for the root path:

• Select Code > Commits from left nav

⚠️ the list of commits uses mock data and would not change when file path gets updated. to validate this change, i confirm the link generated from the new breadcrumb matches the existing one (when FF is off) 👍

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #551400 (closed)