buffer: fix `ENSURE_SIZE` macro referencing wrong variable

While the `ENSURE_SIZE` macro gets a reference to both the buffer that
is to be resized and a new size, we were not consistently referencing
the passed buffer, but instead a variable `buf`, which is not passed in.
Funnily enough, we never noticed because our buffers seem to always be
named `buf` whenever the macro was being used.

Fix the macro by always using the passed-in buffer. While at it, add
braces around all mentions of passed-in variables as should be done with
macros to avoid subtle errors.

Found-by: Edward Thompson

src/buffer.c

@@ -18,7 +18,7 @@ char git_buf__initbuf[1];
 char git_buf__oom[1];
 
 #define ENSURE_SIZE(b, d) \
-	if ((d) > buf->asize && git_buf_grow(b, (d)) < 0)\
+	if ((d) > (b)->asize && git_buf_grow((b), (d)) < 0)\
 		return -1;