git-diff: resurrect the traditional empty "diff --git" behaviour

The warning message to suggest "Consider running git-status" from
"git-diff" that we experimented with during the 1.5.3 cycle turns
out to be a bad idea.  It robbed cache-dirty information from people
who valued it, while still asking users to run "update-index --refresh".
It was hoped that the new behaviour would at least have some educational
value, but not showing the cache-dirty paths like before meant that the
user would not even know easily which paths were cache-dirty, and it
made the need to refresh the index look like even more unnecessary chore.

This commit reinstates the traditional behaviour, but with a twist.

By default, the empty "diff --git" output is totally squelched out
from "git diff" output.  At the end of the command, it automatically
runs "update-index --refresh" as needed, without even bothering the
user.  In other words, people who do not care about the cache-dirtyness
do not even have to see the warning.

The traditional behaviour to see the stat-dirty output and to bypassing
the overhead of content comparison can be specified by setting the
configuration variable diff.autorefreshindex to false.

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

cache.h

@@ -594,6 +594,9 @@ extern char *convert_to_git(const char *path, const char *src, unsigned long *si
 extern char *convert_to_working_tree(const char *path, const char *src, unsigned long *sizep);
 extern void *convert_sha1_file(const char *path, const unsigned char *sha1, unsigned int mode, enum object_type *type, unsigned long *size);
 
+/* diff.c */
+extern int diff_auto_refresh_index;
+
 /* match-trees.c */
 void shift_tree(const unsigned char *, const unsigned char *, unsigned char *, int);