Implement a new 'diff-ignore-content-type' runtime configuration
variable which saves folks from having to include --force when trying
to see differences of binary files.  This is handy when Subversion is
configured via 'diff-cmd' to use an external diff tool that can handle
binary diffs.

NOTE: This option is parsed in 'svn' (not down in the client layer)
      and used to control an existing boolean function parameter.

* subversion/include/svn_config.h
  (SVN_CONFIG_OPTION_DIFF_IGNORE_CONTENT_TYPE): New configuration 
    token #define.

* subversion/svn/diff-cmd.c
  (svn_cl__diff): Check the runtime configuration area to see if diff
    operations should ignore the content-type of files.

* subversion/libsvn_subr/config_file.c
  (svn_config_ensure): Add runtime configuration file template matter
    for the new 'diff-ignore-content-type' option.

* subversion/tests/cmdline/diff_tests.py
  (diff_force): Tweak this test to try the new runtime config option
    as well as '--force', verifying that both allow the display of
    binary diffs.

* CHANGES
  Note this change.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1478826 13f79535-47bb-0310-9956-ffa450edef68