Paper bag fix diff invocation in 'git stash show'

Paper bag fix diff invocation in 'git stash show'

In 89d750bf6fa025edeb31ad258cdd09a27a5c02fa I got a little too
aggressive with changing "git diff" to "git diff-tree".  This is
shown to the user, who expects to see a full diff on their console,
and will want to see the output of their custom diff drivers (if
any) as the whole point of this call site is to show the diff to
the end-user.

Noticed by Johannes Sixt <redacted>.

Signed-off-by: Shawn O. Pearce <redacted>