Bump core.deltaBaseCacheLimit to 96m

Bump core.deltaBaseCacheLimit to 96m

The default of 16m causes serious thrashing for large delta chains
combined with large files.

Here are some benchmarks (pu variant of git blame):

time git blame -C src/xdisp.c >/dev/null

for a repository of Emacs repacked with git gc --aggressive (v1.9,
resulting in a window size of 250) located on an SSD drive.  The file in
question has about 30000 lines, 1Mb of size, and a history with about
2500 commits.

    16m (previous default):
    real	3m33.936s
    user	2m15.396s
    sys	1m17.352s

    32m:
    real	3m1.319s
    user	2m8.660s
    sys	0m51.904s

    64m:
    real	2m20.636s
    user	1m55.780s
    sys	0m23.964s

    96m:
    real	2m5.668s
    user	1m50.784s
    sys	0m14.288s

    128m:
    real	2m4.337s
    user	1m50.764s
    sys	0m12.832s

    192m:
    real	2m3.567s
    user	1m49.508s
    sys	0m13.312s

Signed-off-by: David Kastrup <dak@gnu.org>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>