Some improvements for the buflist implementation

This should make the buflist algorithm faster.
Also there is an alternative implementation in branch 535 available,
which avoids looping over the complete range, I'll stay with the current
approach, as it does not depend on BufAdd/BufDelete autocommands.

details:
instead of testing for buflisted() and bufexists() we only test for
buflisted() because, this also tests for the existence of the buffer.
Also instead of a second loop of the exclude patterns, we'll join all
of them together with '\|' and check if they match the current buffer.
The rest of the conditions have been joined into a single condition.

This together made up an improvement of
Orig:
FUNCTION  airline#extensions#tabline#buflist#list()
Called 94 times
Total time:   0.267305
 Self time:   0.267305

New:
FUNCTION  airline#extensions#tabline#buflist#list()
Called 85 times
Total time:   0.124572
 Self time:   0.124572

autoload/airline/extensions/tabline/buflist.vim

@@ -14,26 +14,24 @@ function! airline#extensions#tabline#buflist#list()
   endif
 
   let buffers = []
-  let cur = bufnr('%')
+  " If this is too slow, we can switch to a different algorithm.
+  " Basically branch 535 already does it, but since it relies on
+  " BufAdd autocommand, I'd like to avoid this if possible.
   for nr in range(1, bufnr('$'))
-    if buflisted(nr) && bufexists(nr)
-      let toadd = 1
-      for ex in s:excludes
-        if match(bufname(nr), ex) >= 0
-          let toadd = 0
-          break
+    if buflisted(nr)
+      " Do not add to the bufferlist, if either
+      " 1) buffername matches exclude pattern
+      " 2) buffer is a quickfix buffer
+      " 3) exclude preview windows (if 'bufhidden' == wipe
+      "    and 'buftype' == nofile
+      if (!empty(s:excludes) && match(bufname(nr), join(s:excludes, '\|')) > -1) ||
+            \ (getbufvar(nr, 'current_syntax') == 'qf') ||
+            \  (s:exclude_preview && getbufvar(nr, '&bufhidden') == 'wipe'
+            \  && getbufvar(nr, '&buftype') == 'nofile')
+        continue
       endif
-      endfor
-      if getbufvar(nr, 'current_syntax') == 'qf'
-        let toadd = 0
-      endif
-      if s:exclude_preview && getbufvar(nr, '&bufhidden') == 'wipe' && getbufvar(nr, '&buftype') == 'nofile'
-        let toadd = 0
-      endif
-      if toadd
       call add(buffers, nr)
     endif
-    endif
   endfor
 
   let s:current_buffer_list = buffers