Backport of the code limiting the time spent walking the node history
when committing a file.

As it turns out, some users make scripted commits that not only create
deep histories for individual files but create many of those files with
the exact same depths of history, i.e. number of changes.

Without this patch, committing the e.g. 65536th change to this set of
files will take hours instead of the usual seconds.  With this patch
we are down to a few minutes.

* subversion/libsvn_fs_fs/fs_fs.c
  (SVN_FS_FS_MAX_DELTIFICATION_WALK): define limit to the effort we spend
                                      on finding a delta base
  (choose_delta_base): don't attempt walks longer than the defined limit

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.7.x-fsfs-commit@1536790 13f79535-47bb-0310-9956-ffa450edef68