Use handy ALLOC_GROW macro in fast-import when possible

Instead of growing our buffer by hand during the inline variant of
cmd_data() we can save a few lines of code and just use the nifty
new ALLOC_GROW macro already available to us.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

fast-import.c

@@ -1487,12 +1487,7 @@ static void *cmd_data (size_t *size)
 			if (term_len == command_buf.len
 				&& !strcmp(term, command_buf.buf))
 				break;
-			if (sz < (length + command_buf.len)) {
-				sz = sz * 3 / 2 + 16;
-				if (sz < (length + command_buf.len))
-					sz = length + command_buf.len;
-				buffer = xrealloc(buffer, sz);
-			}
+			ALLOC_GROW(buffer, length + command_buf.len, sz);
 			memcpy(buffer + length,
 				command_buf.buf,
 				command_buf.len - 1);