cat_one_file(): make it easy to see that the size variable is initialized

The large `switch` statement makes it a bit impractical to reason about
the code.

One of the code paths can technically lead to using `size` without being
initialized: if the `t` case is taken and the type name is set to the
empty string, we would actually leave `size` unintialized right until we
use it.

Practically, this cannot happen because the
`do_oid_object_info_extended()` function is expected to always populate
the `type_name` if asked for. However, it is quite unnecessary to leave
the code as unwieldy to reason about: Just initialize the variable to 0
and be done with it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

builtin/cat-file.c

@@ -106,7 +106,7 @@ static int cat_one_file(int opt, const char *exp_type, const char *obj_name)
 	struct object_id oid;
 	enum object_type type;
 	char *buf;
-	unsigned long size;
+	unsigned long size = 0;
 	struct object_context obj_context = {0};
 	struct object_info oi = OBJECT_INFO_INIT;
 	unsigned flags = OBJECT_INFO_LOOKUP_REPLACE;