Pristines-on-Demand
-------------------

The purpose of this branch is to try to make the text-bases in the working
copy optional [1][2].


Description of the approach:
----------------------------

The core idea is that we start to maintain the following invariant: only the
modified files have their pristine text-base files available on the disk.

- To avoid having to access the text-base, the "is the file modified?" check
  is performed by calculating the checksum of a file and comparing that to
  what's recorded in the working copy.

- A text-base of the unmodified file is the file itself, appropriately
  detranslated.

- To get into the appropriate state at the beginning of the operation, we walk
  through the current text-base info in the db and check if the corresponding
  working files are modified.  The missing text-bases are fetched using the
  svn_ra layer.  The operations also include a final step during which the
  no longer required text-bases are removed from disk.

- The operations that don't need to access the text-bases (such as "svn ls"
  or the updated "svn st") do not perform this walk and do not synchronize
  the text-base state.


For the production-ready state, it would probably make sense to also:
---------------------------------------------------------------------

 1. Complete the work on ^/subversion/branches/multi-wc-format so that the
    client would work with both the new and old working copy formats, for
    a seamless user experience and better compatibility.

    (Done in r1897946)

 2. For the new working copy format, incorporate a switch to a different
    checksum type without known collisions instead of SHA-1.


References:
-----------

[1] https://issues.apache.org/jira/browse/SVN-525
[2] https://lists.apache.org/thread.html/r40aa795b9e0c1aabb392c43b64e540432a31e22d8b0d9de7202b4950%40%3Cdev.subversion.apache.org%3E