buffer: consistently use `ENSURE_SIZE` to grow buffers on-demand

The `ENSURE_SIZE` macro can be used to grow a buffer if its currently
allocated size does not suffice a required target size. While most of
the code already uses this macro, the `git_buf_join` and `git_buf_join3`
functions do not yet use it. Due to the macro first checking whether we
have to grow the buffer at all, this has the benefit of saving a
function call when it is not needed. While this is nice to have, it will
probably not matter at all performance-wise -- instead, this only serves
for consistency across the code.

src/buffer.c

@@ -724,9 +724,7 @@ int git_buf_join(
 	GITERR_CHECK_ALLOC_ADD(&alloc_len, strlen_a, strlen_b);
 	GITERR_CHECK_ALLOC_ADD(&alloc_len, alloc_len, need_sep);
 	GITERR_CHECK_ALLOC_ADD(&alloc_len, alloc_len, 1);
-	if (git_buf_grow(buf, alloc_len) < 0)
-		return -1;
-	assert(buf->ptr);
+	ENSURE_SIZE(buf, alloc_len);
 
 	/* fix up internal pointers */
 	if (offset_a >= 0)
@@ -780,8 +778,7 @@ int git_buf_join3(
 	GITERR_CHECK_ALLOC_ADD(&len_total, len_total, sep_b);
 	GITERR_CHECK_ALLOC_ADD(&len_total, len_total, len_c);
 	GITERR_CHECK_ALLOC_ADD(&len_total, len_total, 1);
-	if (git_buf_grow(buf, len_total) < 0)
-		return -1;
+	ENSURE_SIZE(buf, len_total);
 
 	tgt = buf->ptr;