Extend svn_diff_file_options_t and svn_diff_file_output_unified4() to allow
configuring the context size for file diffs.

* subversion/include/svn_diff.h
  (svn_diff_file_options_t): Add variable.
  (svn_diff_file_output_unified4): Add argument.
  (svn_diff_file_output_unified3): Document legacy behavior.

* subversion/libsvn_client/diff.c
  (diff_content_changed): Update caller.

* subversion/libsvn_diff/deprecated.c
  (svn_diff_file_output_unified3): Update caller.

* subversion/libsvn_diff/diff_file.c
  (svn_diff_file_options_create): Initialize with original default.
  (svn_diff_file_output_unified4): Add argument. Update initialization.

* subversion/svn/conflict-callbacks.c
  (show_diff): Update caller. Use options to document defaults.

* subversion/svnlook/svnlook.c
  (print_diff_tree): Update caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1597983 13f79535-47bb-0310-9956-ffa450edef68