Simplifies some of the "active tab" checks we're doing

Because of the way ExtractPaths works, `params[:format]` wouldn't
necessarily be available at the time this filter was running, and so it
would erroneously redirect to `new_user_session_path`

Takes tree-ish + path as ID

Update docs a bit

Commits#show (plural) is going to be for showing commit history on a
specific path.

-`remove comment` button will not make the text jump anymore.

[ci skip]