* lib/diffseq.h: Problem with diffutils reported by Andreas Schwab
(Bug#24715). The simplest solution is to restore the
TOO_EXPENSIVE heuristic that I added to GNU diff in 1993, while
using a higher threshold to avoid Bug#16848 on smaller files.
* lib/diffseq.h (struct context): Restore member too_expensive.
(struct partition): Restore members lo_minimal, hi_minimal.
(diag, compareseq): Restore arg find_minimal. All uses changed.
(diag): Restore the TOO_EXPENSIVE heuristic that I added back in
1993 to make 'diff' run faster (but not as well) on large inputs,
but use a threshold of 4096 instead of the old 256.
* lib/fstrcmp.c (strcmp_bounded):
* lib/git-merge-changelog.c (compute_differences):
Adjust to diffseq.h changes.