*.[ch] *_INIT macros: use { 0 } for a "zero out" idiom

In C it isn't required to specify that all members of a struct are
zero'd out to 0, NULL or '\0', just providing a "{ 0 }" will
accomplish that.

Let's also change code that provided N zero'd fields to just
provide one, and change e.g. "{ NULL }" to "{ 0 }" for
consistency. I.e. even if the first member is a pointer let's use "0"
instead of "NULL". The point of using "0" consistently is to pick one,
and to not have the reader wonder why we're not using the same pattern
everywhere.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diff.c b/diff.c
index c8f530f..861282d 100644
--- a/diff.c
+++ b/diff.c
@@ -775,13 +775,13 @@ struct emitted_diff_symbol {
 	int indent_width; /* The visual width of the indentation */
 	enum diff_symbol s;
 };
-#define EMITTED_DIFF_SYMBOL_INIT {NULL}
+#define EMITTED_DIFF_SYMBOL_INIT { 0 }
 
 struct emitted_diff_symbols {
 	struct emitted_diff_symbol *buf;
 	int nr, alloc;
 };
-#define EMITTED_DIFF_SYMBOLS_INIT {NULL, 0, 0}
+#define EMITTED_DIFF_SYMBOLS_INIT { 0 }
 
 static void append_emitted_diff_symbol(struct diff_options *o,
 				       struct emitted_diff_symbol *e)