log-tree: use FLEX_ARRAY in name_decoration

We are already using the flex-array technique; let's
annotate it with our usual FLEX_ARRAY macro. Besides being
more readable, this is slightly more efficient on compilers
that understand flex-arrays.

Note that we need to bump the allocation in add_name_decoration,
which did not explicitly add one byte for the NUL terminator
of the string we are putting into the flex-array (it did not
need to before, because the struct itself was over-allocated
by one byte).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -30,7 +30,7 @@ extern const char *commit_type;
 struct name_decoration {
 	struct name_decoration *next;
 	int type;
-	char name[1];
+	char name[FLEX_ARRAY];
 };
 
 enum decoration_type {