patch 9.0.0786: user command does not get number from :tab modifier

Problem:    User command does not get number from :tab modifier.
Solution:   Include the number. (closes #11393, closes #6801)
diff --git a/src/testdir/test_usercommands.vim b/src/testdir/test_usercommands.vim
index 961aeb2..f8cc1f5 100644
--- a/src/testdir/test_usercommands.vim
+++ b/src/testdir/test_usercommands.vim
@@ -81,6 +81,19 @@
   call assert_equal('silent!', g:mods)
   tab MyCmd
   call assert_equal('tab', g:mods)
+  0tab MyCmd
+  call assert_equal('0tab', g:mods)
+  tab split
+  tab MyCmd
+  call assert_equal('tab', g:mods)
+  1tab MyCmd
+  call assert_equal('1tab', g:mods)
+  tabprev
+  tab MyCmd
+  call assert_equal('tab', g:mods)
+  2tab MyCmd
+  call assert_equal('2tab', g:mods)
+  2tabclose
   topleft MyCmd
   call assert_equal('topleft', g:mods)
   to MyCmd