Fix inline blame previous-blame link on relative URL installs

What does this MR do and why?

Fixes a bug in the inline blame view where the "View blame prior to this change" button links to a URL missing the relative URL root prefix, resulting in a 404 on instances installed under a relative path (e.g., https://example.com/gitlab).

The root cause was in previousBlameUrl computed prop in app/assets/javascripts/vue_shared/components/source_viewer/components/blame_commit_info.vue. It built the href using joinPaths('/', projectPath, '-/blob', parentSha, previousPath), which produces a root-relative path that ignores gon.relative_url_root.

The fix uses the projectBlobPath route helper from ~/lib/utils/path_helpers/repository instead. Route helpers are globally configured with script_name: gon.relative_url_root (in app/assets/javascripts/behaviors/configure_path_helpers.js), so the relative URL root is automatically applied. The ?blame=1 query parameter is passed as a route option.

References

Resolves #627734 (closed)

Related: !245993 (merged)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Configure a relative URL root. In GDK, set relative_url_root: /gitlab in gdk.yml, then run gdk reconfigure && gdk restart. For Omnibus, set external_url "https://example.com/gitlab" in gitlab.rb.
2. Open any text file with more than one commit in its history.
3. Select Blame in the file header to enable inline blame.
4. Hover a blame entry and check the "View blame prior to this change" button (doc-versions icon). Its link should include the relative URL root and open the previous version's blame view instead of a 404.

MR acceptance checklist

This MR is expected to adhere to the acceptance checklist. The checklist has been evaluated.