* buffer.h (struct buffer_text): New field chars_modiff.

* buffer.h (struct buffer_text): New field chars_modiff.
        (CHARS_MODIFF, BUF_CHARS_MODIFF): New macros.
        * buffer.c (Fbuffer_chars_modified_tick): New function returning
        value of BUF_CHARS_MODIFF.
        (syms_of_buffer): Defsubr it.
        (Fget_buffer_create): Initialize BUF_CHARS_MODIFF.
        * insdel.c (modify_region): New argument preserve_chars_modiff.
        Set CHARS_MODIFF to MODIFF provided preserve_chars_modiff is zero.
        (insert_1_both, insert_from_string_1, insert_from_buffer_1)
        (adjust_after_replace, adjust_after_replace_noundo)
        (replace_range, replace_range_2, del_range_2): Reset CHARS_MODIFF.
        * lisp.h (modify_region): Add fourth argument in extern.
        * casefiddle.c (casify_region): Call modify_region with fourth
        argument zero to assert that CHARS_MODIFF is updated.
        * editfns.c (Fsubst_char_in_region, Ftranslate_region_internal)
        (Ftranspose_regions): Likewise.
        * textprop.c (Fadd_text_properties, Fset_text_properties)
        (Fremove_text_properties, Fremove_list_of_text_properties):
        Call modify_region with fourth argument 1 to avoid that
        CHARS_MODIFF is updated.