patch 8.1.0377: xdiff doesn't use the Vim memory allocation functions

Problem:    Xdiff doesn't use the Vim memory allocation functions.
Solution:   Change the xdl_ defines.  Check for out-of-memory.  Rename
            "ignored" to "vim_ignored".
diff --git a/src/channel.c b/src/channel.c
index 793dbaa..aa0a0d3 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -1858,7 +1858,7 @@
     {
 	ch_log_lead(lead, channel, part);
 	fprintf(log_fd, "'");
-	ignored = (int)fwrite(buf, len, 1, log_fd);
+	vim_ignored = (int)fwrite(buf, len, 1, log_fd);
 	fprintf(log_fd, "'\n");
     }
     return OK;
@@ -2388,7 +2388,7 @@
 
     u_sync(TRUE);
     /* ignore undo failure, undo is not very useful here */
-    ignored = u_save(lnum - empty, lnum + 1);
+    vim_ignored = u_save(lnum - empty, lnum + 1);
 
     if (empty)
     {
@@ -3730,7 +3730,7 @@
     {
 	ch_log_lead("SEND ", channel, part);
 	fprintf(log_fd, "'");
-	ignored = (int)fwrite(buf_arg, len_arg, 1, log_fd);
+	vim_ignored = (int)fwrite(buf_arg, len_arg, 1, log_fd);
 	fprintf(log_fd, "'\n");
 	fflush(log_fd);
 	did_log_msg = TRUE;