Fix a performance regression of WC-NG vs. WC 1.0:  When performing a
fulltext comparison of WORKING vs BASE, use unbuffered files.

For cached data (e.g. commit after 'svn status'), this increases the
throughput by ~70% for unchanged sections of the respective files.

We will *always* use unbuffered APR files as the basis for prestine
file streams.  More complex operations such as newline translations
use their own buffer layer.  Therefore, the underlying APR file should
never be subject to fine-grained reads.

* subversion/libsvn_wc/questions.c
  (compare_and_verify): Read WORKING file as unbuffered stream, remove
                        redundant conditions and duplicated code.

* subversion/libsvn_wc/wc_db_pristine.c
  (pristine_read_txn): Return prestine file contents as unbuffered streams.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1586214 13f79535-47bb-0310-9956-ffa450edef68