feat(commands): diff the current buffer against its last save

[barrettruth]

Problem

:Diff files {left} is documented as taking the current buffer as its new side, and that is how it was described when the subcommand was added, but the implementation resolves that buffer to its name and then reads the name back from disk. Both sides of a single-path invocation are therefore disk reads: unsaved changes are invisible, and naming the current file compares it to itself and always reports no changes. That also contradicts the claim that the form mirrors :Diff {rev}, which does substitute live buffer content for the worktree endpoint. Nothing in the command surface spells "compare this buffer to its last save".

Solution

Apply the rule :Diff already follows to :Diff files: a named path is read from disk, and a side the user did not name is the current buffer. The new side of a single-path invocation becomes the buffer's live content, and a bare :Diff files also defaults the old side to that buffer's file on disk, so it compares the buffer to its last save. Two named paths behave exactly as before.

A buffer-backed side reads as buffer:{name} in the view name, the list title, and messages, while the diff header keeps real paths so hunk and path parsing are untouched. The files source records the buffer it was opened against, so reloading the view re-reads that buffer instead of quietly reverting to disk, and falls back to the path once the buffer is gone. Difftastic takes the content-based span maps for a buffer-backed side, since the path-based ones can only see disk, and a named buffer that has never been written is no longer refused as unreadable.

Closes #429.