patch 8.1.0794: white space before " -Ntabmove" causes problems

Problem:    White space before " -Ntabmove" causes problems.
Solution:   Skip whitespace. (Ozaki Kiichi, closes #3841)

src/ex_docmd.c

@@ -7588,7 +7588,7 @@ get_tabpage_arg(exarg_T *eap)
 	else
 	{
 	    tab_number = eap->line2;
-	    if (!unaccept_arg0 && **eap->cmdlinep == '-')
+	    if (!unaccept_arg0 && *skipwhite(*eap->cmdlinep) == '-')
 	    {
 		--tab_number;
 		if (tab_number < unaccept_arg0)

src/testdir/test_tabpage.vim

@@ -105,6 +105,14 @@ function Test_tabpage()
   call assert_equal(4, tabpagenr())
   7tabmove 5
   call assert_equal(5, tabpagenr())
+  -tabmove
+  call assert_equal(4, tabpagenr())
+  +tabmove
+  call assert_equal(5, tabpagenr())
+  -2tabmove
+  call assert_equal(3, tabpagenr())
+  +3tabmove
+  call assert_equal(6, tabpagenr())
 
   " The following are a no-op
   norm! 2gt

src/version.c

@@ -791,6 +791,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    794,
 /**/
     793,
 /**/