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.h

@@ -17,6 +17,15 @@ enum getf_values {
   GETF_SWITCH  = 0x04, // respect 'switchbuf' settings when jumping
 };
 
+// Return values of getfile()
+enum getf_retvalues {
+  GETFILE_ERROR       = 1,    // normal error
+  GETFILE_NOT_WRITTEN = 2,    // "not written" error
+  GETFILE_SAME_FILE   = 0,    // success, same file
+  GETFILE_OPEN_OTHER  = -1,   // success, opened another file
+  GETFILE_UNUSED      = 8
+};
+
 // Values for buflist_new() flags
 enum bln_values {
   BLN_CURBUF = 1,   // May re-use curbuf for new buffer