Add commandline support for configuring the context size of our
unified diff output. This allows
$ svn diff -x -U9 WC

Note: this patch doesn't update the help output for -x, which
will need a separate patch.

* subversion/include/svn_diff.h
  (svn_diff_file_options_parse): Document -U and --context.

* subversion/libsvn_diff/diff_file.c
  (diff_options): Add context.
  (svn_diff_file_options_parse): Handle context option.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1603847 13f79535-47bb-0310-9956-ffa450edef68