Ensure inactive tabs are force-updated after FocusGained.

On FocusGained the current tab is force-updated.  If there are any other
tabs, we want to make sure they are also force-updated when the user
next switches to them.  To do this we set a flag which is read and
cleared on TabEnter.

See #525.

plugin/gitgutter.vim

@@ -173,6 +173,16 @@ nnoremap <silent> <Plug>GitGutterPreviewHunk :GitGutterPreviewHunk<CR>
 
 " }}}
 
+function! s:flag_inactive_tabs()
+  let active_tab = tabpagenr()
+  let last_tab = tabpagenr('$')
+  for i in range(1, last_tab)
+    if i != active_tab
+      call settabvar(i, 'gitgutter_force', 1)
+    endif
+  endfor
+endfunction
+
 " Autocommands {{{
 
 augroup gitgutter
@@ -183,7 +193,12 @@ augroup gitgutter
   autocmd BufEnter *
         \ if exists('t:gitgutter_didtabenter') && t:gitgutter_didtabenter |
         \   let t:gitgutter_didtabenter = 0 |
-        \   call gitgutter#all(!g:gitgutter_terminal_reports_focus) |
+        \   let force = !g:gitgutter_terminal_reports_focus |
+        \   if exists('t:gitgutter_force') && t:gitgutter_force |
+        \     let t:gitgutter_force = 0 |
+        \     let force = 1 |
+        \   endif |
+        \   call gitgutter#all(force) |
         \ else |
         \   call gitgutter#init_buffer(bufnr('')) |
         \   call gitgutter#process_buffer(bufnr(''), !g:gitgutter_terminal_reports_focus) |
@@ -197,7 +212,7 @@ augroup gitgutter
   "   vim -o file1 file2
   autocmd VimEnter * if winnr() != winnr('$') | call gitgutter#all(0) | endif
 
-  autocmd FocusGained * call gitgutter#all(1)
+  autocmd FocusGained * call gitgutter#all(1) | call s:flag_inactive_tabs()
 
   autocmd ColorScheme * call gitgutter#highlight#define_sign_column_highlight() | call gitgutter#highlight#define_highlights()