patch 8.2.0054: :diffget and :diffput don't have good completion

Problem:    :diffget and :diffput don't have good completion.
Solution:   Add proper completion. (Dominique Pelle, closes #5409)
diff --git a/src/vim.h b/src/vim.h
index a01d4b2..c4c649c 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -801,6 +801,7 @@
 #define EXPAND_MESSAGES		46
 #define EXPAND_MAPCLEAR		47
 #define EXPAND_ARGLIST		48
+#define EXPAND_DIFF_BUFFERS	49
 
 // Values for exmode_active (0 is no exmode)
 #define EXMODE_NORMAL		1
@@ -829,6 +830,7 @@
 #define WILD_IGNORE_COMPLETESLASH   0x400
 #define WILD_NOERROR		    0x800  // sets EW_NOERROR
 #define WILD_BUFLASTUSED	    0x1000
+#define BUF_DIFF_FILTER		    0x2000
 
 // Flags for expand_wildcards()
 #define EW_DIR		0x01	// include directory names