main: Make :AirlineRefresh! skip recrecrating highlighting groups

When `g:skip_empty_sections` is set, `:AirlineRefresh` can be called
very often and cause slow down, because it forcefully re-creates the
highlighting groups. This is mostly not needed for the redraw to happen,
therefore, add the `<bang>` attribute to the command, making it skip to
re-create the highlighting groups and have all extensions that rely on a
forced update use the `!` form.

Should be much fast then.

fixes #1908

autoload/airline/extensions/ale.vim

@@ -124,6 +124,6 @@ endfunction
 
 function! s:ale_refresh()
   if get(g:, 'airline_skip_empty_sections', 0)
-    exe ':AirlineRefresh'
+    exe ':AirlineRefresh!'
   endif
 endfunction

autoload/airline/extensions/languageclient.vim

@@ -22,7 +22,7 @@ let s:diagnostics = {}
 
 function! s:languageclient_refresh()
   if get(g:, 'airline_skip_empty_sections', 0)
-    exe ':AirlineRefresh'
+    exe ':AirlineRefresh!'
   endif
 endfunction
 

autoload/airline/extensions/whitespace.vim

@@ -165,7 +165,7 @@ function! s:ws_refresh()
   endif
   unlet! b:airline_whitespace_check
   if get(g:, 'airline_skip_empty_sections', 0)
-    exe ':AirlineRefresh'
+    exe ':AirlineRefresh!'
   endif
   let b:airline_ws_changedtick = b:changedtick
 endfunction

doc/airline.txt

@@ -257,8 +257,9 @@ COMMANDS                                                  *airline-commands*
 :AirlineToggle                                              *:AirlineToggle*
   Toggles between the standard 'statusline' and airline.
 
-:AirlineRefresh                                            *:AirlineRefresh*
-  Refreshes all highlight groups and redraws the statusline.
+:AirlineRefresh[!]                                            *:AirlineRefresh*
+  Refreshes all highlight groups and redraws the statusline. With the '!'
+  attribute, skips refreshing the highlighting groups.
 
 :AirlineExtensions                                       *:AirlineExtensions*
   Shows the status of all available airline extensions.

plugin/airline.vim

@@ -195,14 +195,18 @@ function! s:airline_theme(...)
   endif
 endfunction
 
-function! s:airline_refresh()
+function! s:airline_refresh(...)
+  " a:1, fast refresh, do not reload the theme
+  let fast=!empty(get(a:000, 0, 0))
   if !exists("#airline")
     " disabled
     return
   endif
   call airline#util#doautocmd('AirlineBeforeRefresh')
   call airline#highlighter#reset_hlcache()
-  call airline#load_theme()
+  if !fast
+    call airline#load_theme()
+  endif
   call airline#update_statusline()
   call airline#update_tabline()
 endfunction
@@ -255,7 +259,7 @@ endfunction
 command! -bar -nargs=? -complete=customlist,<sid>get_airline_themes AirlineTheme call <sid>airline_theme(<f-args>)
 command! -bar AirlineToggleWhitespace call airline#extensions#whitespace#toggle()
 command! -bar AirlineToggle  call s:airline_toggle()
-command! -bar AirlineRefresh call s:airline_refresh()
+command! -bar -bang AirlineRefresh call s:airline_refresh(<q-bang>)
 command! AirlineExtensions   call s:airline_extensions()
 
 call airline#init#bootstrap()