Add command support for dropping a stashed commit

Add command support for dropping a stashed commit

This extracts the logic for writing to the reflog from
RefDirectory into a new ReflogWriter class.  This class
creates a public API for writing reflog entries similar
to ReflogReader for reading reflog entries.

The new command supports rewriting the stash's log to remove
a configured entry followed by updating the stash ref to
the value at the bottom of the newly written log.

Change-Id: Icfcbc70e838666769a742a94196eb8dc9c7efcc7
Signed-off-by: Chris Aniszczyk <zx@twitter.com>