Allow multiple leading colons before and after modifiers for 'inccommand'

src/nvim/ex_docmd.c

@@ -10216,10 +10216,13 @@ bool cmd_can_preview(char_u *cmd)
     return false;
   }
 
+  // Ignore additional colons at the start...
+  cmd = skip_colon_white(cmd, true);
+
   // Ignore any leading modifiers (:keeppatterns, :verbose, etc.)
   for (int len = modifier_len(cmd); len != 0; len = modifier_len(cmd)) {
     cmd += len;
-    cmd = skipwhite(cmd);
+    cmd = skip_colon_white(cmd, true);
   }
 
   exarg_T ea;

test/functional/ui/inccommand_spec.lua

@@ -779,6 +779,13 @@ describe(":substitute, inccommand=split", function()
       {15:~                             }|
       :silent tabedit %s/tw/to^      |
     ]])
+    feed('<Esc>')
+
+    -- leading colons
+    feed(':::%s/tw/to')
+    screen:expect{any=[[{12:to}o lines]]}
+    feed('<Esc>')
+    screen:expect{any=[[two lines]]}
   end)
 
   it('shows split window when typing the pattern', function()