stash: Use main view infrastructure for the stash view

Change the output to use "--pretty=raw", which is compatible with the
main view. This simplifies the parsing of the 'stash{X}' ID by using the
emitted 'Reflog' line and displays the stash date for free.

The reference to the stash is stored as the commit->id for easy access
when updating the status line (view->ref).

Fixes #169
stash: Refactor ID drawing method

Simpify the main method (draw_id) and add custom method for more
advanced requirements.
stash: Ensure that the whole stash@{_} text is shown

Fixes #169
stash: Highlight the diffstat of combined diffs

Fixes #169