Fix appending to existing file feature test case

Signed-off-by: Pekka Enberg <penberg@iki.fi>
extent: SHA1 hashing

Store a SHA1 hash in of extent content in the extent header which can be
used for data integrity checks.  Note that this changes the disk format
and makes pstore incompatible with files created by earlier versions.

Signed-off-by: Pekka Enberg <penberg@iki.fi>