diff: deprecate -q option to diff-files

This reimplements the ancient "-q" option to "git diff-files" that
was inherited from "show-diff -q" in terms of "--diff-filter=d".  We
will be deprecating the "-q" option, so let's issue a warning when
we do so.

Incidentally this also tentatively fixes "git diff --no-index" to
honor "-q" and hide deletions; the use will get the same warning.

We should remove the support for "-q" in a future version but it is
not that urgent.

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

diff.h

@@ -341,6 +341,8 @@ extern int parse_rename_score(const char **cp_p);
 
 extern long parse_algorithm_value(const char *value);
 
+extern void handle_deprecated_show_diff_q(struct diff_options *);
+
 extern int print_stat_summary(FILE *fp, int files,
 			      int insertions, int deletions);
 extern void setup_diff_pager(struct diff_options *);