Link blobs to their history and back

Nothing in a commits payload says whether the path it lists is a file or
a directory: currentBlobPath, despite the name, is only the path filter,
and is set for a directory just the same. GitHub's own UI cannot tell
either, and points its browse button at /blob/<oid>/<path> so that the
redirect resolves a file to its blob. We do the same, and the blob route
already turns that redirect into one of our own tree URLs.

Breadcrumbs now stay in the view they were clicked from, so walking up
from a history lands in the history of each parent directory rather than
in its listing. The last crumb remains the page you are already on.

A path with no commits now says so rather than rendering an empty page.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R7pRKdmTvNDgT4ZXrwkbH8

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