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>

Documentation/config.txt

@@ -489,7 +489,7 @@ core.deltaBaseCacheLimit::
 	to avoid unpacking and decompressing frequently used base
 	objects multiple times.
 +
-Default is 16 MiB on all platforms.  This should be reasonable
+Default is 96 MiB on all platforms.  This should be reasonable
 for all users/operating systems, except on the largest projects.
 You probably do not need to adjust this value.
 +

environment.c

@@ -37,7 +37,7 @@ int core_compression_seen;
 int fsync_object_files;
 size_t packed_git_window_size = DEFAULT_PACKED_GIT_WINDOW_SIZE;
 size_t packed_git_limit = DEFAULT_PACKED_GIT_LIMIT;
-size_t delta_base_cache_limit = 16 * 1024 * 1024;
+size_t delta_base_cache_limit = 96 * 1024 * 1024;
 unsigned long big_file_threshold = 512 * 1024 * 1024;
 const char *pager_program;
 int pager_use_color = 1;