restore original directory change detection. also resolves #474.

autoload/airline/extensions/branch.vim

@@ -2,7 +2,6 @@
 " vim: et ts=2 sts=2 sw=2
 
 let s:has_fugitive = exists('*fugitive#head')
-let s:has_fugitive_detect = 0 " TODO: figure out alternative
 let s:has_lawrencium = exists('*lawrencium#statusline')
 let s:has_vcscommand = get(g:, 'airline#extensions#branch#use_vcscommand', 0) && exists('*VCSCommandGetStatusLine')
 
@@ -10,8 +9,30 @@ if !s:has_fugitive && !s:has_lawrencium && !s:has_vcscommand
   finish
 endif
 
+let s:git_dirs = {}
+function! s:get_git_branch(path)
+  if has_key(s:git_dirs, a:path)
+    return s:git_dirs[a:path]
+  endif
+
+  let dir = fugitive#extract_git_dir(a:path)
+  if empty(dir)
+    let name = ''
+  else
+    try
+      let line = join(readfile(dir . '/HEAD'))
+      let name = strpart(line, 16)
+    catch
+      let name = ''
+    endtry
+  endif
+
+  let s:git_dirs[a:path] = name
+  return name
+endfunction
+
 function! airline#extensions#branch#head()
-  if exists('b:airline_head')
+  if exists('b:airline_head') && !empty(b:airline_head)
     return b:airline_head
   endif
 
@@ -20,9 +41,8 @@ function! airline#extensions#branch#head()
   if s:has_fugitive && !exists('b:mercurial_dir')
     let b:airline_head = fugitive#head()
 
-    if empty(b:airline_head) && s:has_fugitive_detect && !exists('b:git_dir')
-      call fugitive#detect(getcwd())
-      let b:airline_head = fugitive#head()
+    if empty(b:airline_head) && !exists('b:git_dir')
+      let b:airline_head = s:get_git_branch(getcwd())
     endif
   endif
 
@@ -84,5 +104,5 @@ function! airline#extensions#branch#init(ext)
   call airline#parts#define_function('branch', 'airline#extensions#branch#get_head')
 
   autocmd BufReadPost * unlet! b:airline_file_in_root
-  autocmd CursorHold * unlet! b:airline_head
+  autocmd CursorHold,ShellCmdPost,CmdwinLeave * unlet! b:airline_head
 endfunction