Complete the background pack handling code in FSFS format7.

So far, we only ensured internal consistency (i.e. index files
pack status matches rev pack status).  Now, we actually re-read
the pack status after we detected a background pack run and make
this transparent to the user.

* subversion/libsvn_fs_fs/index.c
  (retry_open_l2p_index):  Update struct members even if FILE has
                           not been previously used / opened.
  (svn_fs_fs__item_offset): Re-read pack status after we discovered a
                            mismatch.  Only then may a retry succeed.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1553051 13f79535-47bb-0310-9956-ffa450edef68