Add FSFS index checksum verification code to 'svnadmin verify'.

We don't verify the index data against the checksums on every
access as there is plenty of cross-verification within the indexes,
between them as well as between index and rev data.  Only if some
inconsistency has been detected and the user wants to trace down
the source (using 'svnadmin verify'), will we verify that the index
data has not been tampered with.

* subversion/libsvn_fs_fs/verify.c
  (verify_index_checksum,
   verify_index_checksums): New per rev / pack file verification code.
  (verify_index_consistency): Call the new index checksum test before
                              using any of the index contents.

* subversion/tests/libsvn_fs_fs/fs-fs-fuzzy-test.c
  (fuzzing_1_byte_1_rev): Remove the code that would accept certain
                          index modifications.  Accept only case changes
                          in index MD5 digests because they don't affect
                          the validity.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1631598 13f79535-47bb-0310-9956-ffa450edef68