* bootstrap.conf (gnulib_modules): Add c32isspace, c32tolower.
* lib/Makefile.am (noinst_HEADERS): Add mbcel.h.
(libdiffutils_a_SOURCES): Add mbcel.c
* lib/mbcel.c, lib/mbcel.h: New files.
* src/io.c: Include mbcel.h, uchar.h.
(hash): 2nd arg is now hash_value, not merely unsigned char,
since the caller might pass a char32_t now.
(find_and_hash_each_line): Support multi-byte input.
* src/util.c: Include mbcel.h, uchar.h.
(lines_differ): New args S1LEN, S2LEN, needed for mbcel_scan.
Caller changed. Support multi-byte input.
* tests/ignore-case: New file.
* tests/Makefile.am (TESTS): Add it.
* tests/ignore-tab-expansion: Add UTF-8 test.
* tests/init.cfg (require_utf8_locale_): New function.
* tests/side-by-side: Use it. Add a column-counting test.
