build: update gnulib submodule to latest
diff: remove TOO_EXPENSIVE heuristic

Problem reported by Vincent Lefevre in <http://bugs.gnu.org/16848>.
The simplest solution is to remove the TOO_EXPENSIVE heuristic
that I added to GNU diff in 1993.  Although appropriate for
circa-1993 hardware, these days the heuristic seems to be more
trouble than it's worth.
* NEWS: Document this.
* doc/diffutils.texi (Overview): Modernize citations.
Remove mention of TOO_EXPENSIVE heuristic.
* src/analyze.c (diff_2_files): Adjust to TOO_EXPENSIVE-related
API changes in gnulib's diffseq module.
diff: fix bug with -I and overlapping hunks

Problem reported by Vincent Lefevre in <http://bugs.gnu.org/16864>.
* src/context.c (find_hunk): Threshold is CONTEXT only if
the second change is ignorable.
* tests/ignore-matching-lines: New test.
* tests/Makefile.am (TESTS): Add it.
diff, sdiff: minor integer overflow fixes

* src/context.c (find_hunk):
Simplify, now that 2 * context + 1 cannot overflow.
* src/diff.c (main):
* src/sdiff.c (interact):
Don't rely on undefined behavior on signed integer overflow.
* src/diff.c (main): Don't let contexts exceed CONTEXT_MAX.
* src/system.h (CONTEXT_MAX): New macro.