core: log offset pack data accesses happened

In a workload other than "git log" (without pathspec nor any option that
causes us to inspect trees and blobs), the recency pack order is said to
cause the access jump around quite a bit. Add a hook to allow us observe
how bad it is.

"git config core.logpackaccess /var/tmp/pal.txt" will give you the log
in the specified file.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -569,6 +569,9 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.logpackaccess"))
+		return git_config_string(&log_pack_access, var, value);
+
 	if (!strcmp(var, "core.autocrlf")) {
 		if (value && !strcasecmp(value, "input")) {
 			if (core_eol == EOL_CRLF)