Document 'stash clear' recovery via unreachable commits

Document 'stash clear' recovery via unreachable commits

Add an example to the stash documentation that shows how to quickly
find candidate commits among the 'git fsck --unreachable' output.
Unless you have merges of branch names containing WIP, or edit your
merge messages to say WIP, there will be no false positives.

Snippet written by Björn "doener" Steinbrink and me after zepolen_
asked on IRC.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>