Add EndOfBuffer hl group for ~ lines after the last line in buffers

This makes it possible to highlight the lines starting with ~ at the end
of buffers and other elements highlighted using NonText.

As proposed by mhinz at
https://groups.google.com/forum/#!topic/vim_dev/p3de1iU1GXI/discussion

src/nvim/globals.h

@@ -405,7 +405,8 @@ EXTERN int no_check_timestamps INIT(= 0);       /* Don't check timestamps */
 typedef enum {
   HLF_8 = 0         /* Meta & special keys listed with ":map", text that is
                        displayed different from what it is */
-  , HLF_AT          /* @ and ~ characters at end of screen, characters that
+  , HLF_EOB         // after the last line in the buffer
+  , HLF_AT          /* @ characters at end of screen, characters that
                        don't really exist in the text */
   , HLF_D           /* directories in CTRL-D listing */
   , HLF_E           /* error messages */
@@ -451,10 +452,10 @@ typedef enum {
 
 /* The HL_FLAGS must be in the same order as the HLF_ enums!
  * When changing this also adjust the default for 'highlight'. */
-#define HL_FLAGS {'8', '@', 'd', 'e', 'i', 'l', 'm', 'M', 'n', 'N', 'r', 's', \
-                  'S', 'c', 't', 'v', 'V', 'w', 'W', 'f', 'F', 'A', 'C', 'D', \
-                  'T', '-', '>', 'B', 'P', 'R', 'L', '+', '=', 'x', 'X', '*', \
-                  '#', '_', '!', '.', 'o'}
+#define HL_FLAGS {'8', '~', '@', 'd', 'e', 'i', 'l', 'm', 'M', 'n', 'N', 'r', \
+                  's', 'S', 'c', 't', 'v', 'V', 'w', 'W', 'f', 'F', 'A', 'C', \
+                  'D', 'T', '-', '>', 'B', 'P', 'R', 'L', '+', '=', 'x', 'X', \
+                  '*', '#', '_', '!', '.', 'o'}
 
 EXTERN int highlight_attr[HLF_COUNT];       /* Highl. attr for each context. */
 # define USER_HIGHLIGHT

src/nvim/option.c

@@ -365,8 +365,8 @@ typedef struct vimoption {
 # define ISP_LATIN1 (char_u *)"@,161-255"
 
 #define HIGHLIGHT_INIT \
-  "8:SpecialKey,@:NonText,d:Directory,e:ErrorMsg,i:IncSearch,l:Search," \
-  "m:MoreMsg,M:ModeMsg,n:LineNr,N:CursorLineNr,r:Question,s:StatusLine," \
+  "8:SpecialKey,~:EndOfBuffer,@:NonText,d:Directory,e:ErrorMsg,i:IncSearch," \
+  "l:Search,m:MoreMsg,M:ModeMsg,n:LineNr,N:CursorLineNr,r:Question,s:StatusLine," \
   "S:StatusLineNC,c:VertSplit,t:Title,v:Visual,V:VisualNOS,w:WarningMsg," \
   "W:WildMenu,f:Folded,F:FoldColumn,A:DiffAdd,C:DiffChange,D:DiffDelete," \
   "T:DiffText,>:SignColumn,-:Conceal,B:SpellBad,P:SpellCap,R:SpellRare," \

src/nvim/screen.c

@@ -1651,7 +1651,7 @@ static void win_update(win_T *wp)
 
     /* make sure the rest of the screen is blank */
     /* put '~'s on rows that aren't part of the file. */
-    win_draw_end(wp, '~', ' ', row, wp->w_height, HLF_AT);
+    win_draw_end(wp, '~', ' ', row, wp->w_height, HLF_EOB);
   }
 
   /* Reset the type of redrawing required, the window has been updated. */

src/nvim/syntax.c

@@ -5781,6 +5781,7 @@ static char *(highlight_init_both[]) =
       "StatusLine term=reverse,bold cterm=reverse,bold gui=reverse,bold"),
   CENT("StatusLineNC term=reverse cterm=reverse",
       "StatusLineNC term=reverse cterm=reverse gui=reverse"),
+  "default link EndOfBuffer NonText",
   CENT("VertSplit term=reverse cterm=reverse",
       "VertSplit term=reverse cterm=reverse gui=reverse"),
   CENT("DiffText term=reverse cterm=bold ctermbg=Red",