Follow-up to 1547045:  A background pack operation could cause a
concurrent transaction build-up to fail with file-not-found.
(Found by reviewing all calls to index functions for their retry
behavior upon background pack operations).

* subversion/libsvn_fs_fs/cached_data.c
  (svn_fs_fs__check_rep): Fix the detection of background packs.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1592428 13f79535-47bb-0310-9956-ffa450edef68