patch 8.2.0295: highlighting for :s wrong when using different separator

Problem:    Highlighting for :s wrong when using different separator.
Solution:   Use separat argument for search direction and separator. (Rob
            Pilling, closes #5665)
diff --git a/src/testdir/dumps/Test_incsearch_substitute_15.dump b/src/testdir/dumps/Test_incsearch_substitute_15.dump
new file mode 100644
index 0000000..41dd358
--- /dev/null
+++ b/src/testdir/dumps/Test_incsearch_substitute_15.dump
@@ -0,0 +1,4 @@
+|h+0&#ffffff0|e+1&&|l@1|o|/|t|h|e|r+0&&|e| @8
+|~+0#4040ff13&| @18
+|~| @18
+|:+0#0000000&|%|s|;|e|l@1|o|/|t|h|e> @7
diff --git a/src/testdir/test_search.vim b/src/testdir/test_search.vim
index afbea94..6ba87fc 100644
--- a/src/testdir/test_search.vim
+++ b/src/testdir/test_search.vim
@@ -1084,6 +1084,30 @@
   call delete('Xis_subst_script')
 endfunc
 
+func Test_incsearch_highlighting()
+  if !exists('+incsearch')
+    return
+  endif
+  if !CanRunVimInTerminal()
+    throw 'Skipped: cannot make screendumps'
+  endif
+
+  call writefile([
+	\ 'set incsearch hlsearch',
+	\ 'call setline(1, "hello/there")',
+	\ ], 'Xis_subst_hl_script')
+  let buf = RunVimInTerminal('-S Xis_subst_hl_script', {'rows': 4, 'cols': 20})
+  " Give Vim a chance to redraw to get rid of the spaces in line 2 caused by
+  " the 'ambiwidth' check.
+  sleep 300m
+
+  " Using a different search delimiter should still highlight matches
+  " that contain a '/'.
+  call term_sendkeys(buf, ":%s;ello/the")
+  call VerifyScreenDump(buf, 'Test_incsearch_substitute_15', {})
+  call term_sendkeys(buf, "<Esc>")
+endfunc
+
 func Test_incsearch_with_change()
   if !has('timers') || !exists('+incsearch') || !CanRunVimInTerminal()
     throw 'Skipped: cannot make screendumps and/or timers feature and/or incsearch option missing'