patch 9.0.1596: :registers command does not work in sandbox

Problem:    :registers command does not work in sandbox.
Solution:   Add flag to the command. (closes #12473)

Author: julio-b

src/ex_cmds.h

@@ -1286,7 +1286,7 @@ EXCMD(CMD_redrawtabline, "redrawtabline", ex_redrawtabline,
 	EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK,
 	ADDR_NONE),
 EXCMD(CMD_registers,	"registers",	ex_display,
-	EX_EXTRA|EX_NOTRLCOM|EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK,
+	EX_EXTRA|EX_NOTRLCOM|EX_TRLBAR|EX_SBOXOK|EX_CMDWIN|EX_LOCK_OK,
 	ADDR_NONE),
 EXCMD(CMD_resize,	"resize",	ex_resize,
 	EX_RANGE|EX_TRLBAR|EX_WORD1|EX_CMDWIN|EX_LOCK_OK,

src/testdir/test_registers.vim

@@ -51,8 +51,9 @@ func Test_display_registers()
     call feedkeys("i\<C-R>=2*4\n\<esc>")
     call feedkeys(":ls\n", 'xt')
 
-    let a = execute('display')
-    let b = execute('registers')
+    " these commands work in the sandbox
+    let a = execute('sandbox display')
+    let b = execute('sandbox registers')
 
     call assert_equal(a, b)
     call assert_match('^\nType Name Content\n'

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1596,
 /**/
     1595,
 /**/