patch 8.2.4378: incsearch HL broken when calling searchcount in 'tabLine'

Problem:    Incsearch highlight broken when calling searchcount() in 'tabLine'
            function. (Mirko Palmer)
Solution:   Save and restore the incsearch state. (Christian Brabandt,
            closes #9763, closes #9633)
diff --git a/src/testdir/test_search_stat.vim b/src/testdir/test_search_stat.vim
index 15da155..e0820e4 100644
--- a/src/testdir/test_search_stat.vim
+++ b/src/testdir/test_search_stat.vim
@@ -371,6 +371,48 @@
   call delete('Xsearchstatgd')
 endfunc
 
+func Test_search_stat_and_incsearch()
+  CheckScreendump
+
+  let lines =<< trim END
+    call setline(1, ['abc--c', '--------abc', '--abc'])
+    set hlsearch
+    set incsearch
+    set bg=dark
+    set showtabline=2
+
+    function MyTabLine()
+    try
+      let a=searchcount(#{recompute: 1, maxcount: -1})
+      return a.current .. '/' .. a.total
+    catch
+      return ''
+    endtry
+    endfunction
+
+    set tabline=%!MyTabLine()
+  END
+  call writefile(lines, 'Xsearchstat_inc')
+
+  let buf = RunVimInTerminal('-S Xsearchstat_inc', #{rows: 10})
+  call term_sendkeys(buf, "/abc")
+  call TermWait(buf)
+  call VerifyScreenDump(buf, 'Test_searchstat_inc_1', {})
+
+  call term_sendkeys(buf, "\<c-g>")
+  call TermWait(buf)
+  call VerifyScreenDump(buf, 'Test_searchstat_inc_2', {})
+
+  call term_sendkeys(buf, "\<c-g>")
+  call TermWait(buf)
+  call VerifyScreenDump(buf, 'Test_searchstat_inc_3', {})
+
+  call term_sendkeys(buf, "\<esc>:qa\<cr>")
+  call TermWait(buf)
+
+  call StopVimInTerminal(buf)
+  call delete('Xsearchstat_inc')
+endfunc
 
 
 " vim: shiftwidth=2 sts=2 expandtab