vim-patch:8.0.0621: :stag does not respect 'switchbuf'

Problem:    The ":stag" command does not respect 'switchbuf'.
Solution:   Check 'switchbuf' for tag commands that may open a new window.
            (Ingo Karkat, closes vim/vim#1681)  Define macros for the return values
            of getfile().
8ad80dea08

src/nvim/buffer_defs.h

@@ -41,6 +41,7 @@ typedef struct {
 // for kvec
 #include "nvim/lib/kvec.h"
 
+#define GETFILE_SUCCESS(x)    ((x) <= 0)
 #define MODIFIABLE(buf) (buf->b_p_ma)
 
 /*