Fix a negative shift distance caused by an off-by-one error.
It did not affect data correctness but reconstruction efficiency.

* subversion/libsvn_fs_fs/transaction.c
  (shards_spanned): A non-zero walk touches at least the starting shard.
  (choose_delta_base): Fix comment grammar and spelling.

Found by: philip

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1590383 13f79535-47bb-0310-9956-ffa450edef68