Fine-tune the rep delta chain limitation code for FSFS.  The idea is that
with the pack reordering in format 7, the reconstruction performance
("un-deltification") mainly depends on the number of pack files to open.

Chosing a very short base representation is not worth it at all because
the "DELTA" line etc. almost always exceed the potential gains. The linear
deltification section close to HEAD should only be a space saver but not
cause us to read from extra shards.  Finally, only relatively large base
representations justify reading multiple shards / pack files.

In combination with changes to the pack heuristics (later commit), this
patch typically doubles "svnadmin verify" performance at the cost of
1 .. 10% disk space.

* subversion/libsvn_fs_fs/cached_data.h
  (svn_fs_fs__rep_chain_length): Extend the signature such that we also
                                 learn the number of shards / packs along
                                 the rep chain.

* subversion/libsvn_fs_fs/cached_data.c
  (svn_fs_fs__rep_chain_length): Extend the implementation accordingly.

* subversion/libsvn_fs_fs/transaction.c
  (shards_spanned): New utility function.
  (choose_delta_base): Add various additional bounds to the length of the
                       deltification chain.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1554942 13f79535-47bb-0310-9956-ffa450edef68