Trigger User FugitiveChanged on events that change repository

Note that in the context of this event, Fugitive functions will use the
changed repository, which might not be the same as the current buffer's
repository.  Pass bufnr() as the optional dir argument to force the
current buffer.

Closes https://github.com/tpope/vim-fugitive/issues/458

autoload/fugitive.vim

@@ -2323,8 +2323,23 @@ function! s:StageSeek(info, fallback) abort
   return exists('backup') ? backup : line - 1
 endfunction
 
-function! s:ReloadStatus(...) abort
-  call s:ExpireStatus(-1)
+function! s:DoAutocmdChanged(dir) abort
+  let dir = a:dir is# -2 ? '' : FugitiveGitDir(a:dir)
+  if empty(dir) || !exists('#User#FugitiveChanged') || exists('g:fugitive_event')
+    return ''
+  endif
+  try
+    let g:fugitive_event = dir
+    exe s:DoAutocmd('User FugitiveChanged')
+  finally
+    unlet! g:fugitive_event
+    " Force statusline reload with the buffer's Git dir
+    let &ro = &ro
+  endtry
+  return ''
+endfunction
+
+function! s:ReloadStatusBuffer(...) abort
   if get(b:, 'fugitive_type', '') !=# 'index'
     return ''
   endif
@@ -2336,6 +2351,13 @@ function! s:ReloadStatus(...) abort
   return ''
 endfunction
 
+function! s:ReloadStatus(...) abort
+  call s:ExpireStatus(-1)
+  call s:ReloadStatusBuffer(a:0 ? a:1 : line('.'))
+  exe s:DoAutocmdChanged(-1)
+  return ''
+endfunction
+
 let s:last_time = reltime()
 if !exists('s:last_times')
   let s:last_times = {}
@@ -2364,13 +2386,13 @@ function! s:ReloadWinStatus(...) abort
     return
   endif
   if !exists('b:fugitive_reltime')
-    exe s:ReloadStatus()
+    exe s:ReloadStatusBuffer()
     return
   endif
   let t = b:fugitive_reltime
   if reltimestr(reltime(s:last_time, t)) =~# '-\|\d\{10\}\.' ||
         \ reltimestr(reltime(get(s:last_times, s:cpath(s:Dir()), t), t)) =~# '-\|\d\{10\}\.'
-    exe s:ReloadStatus()
+    exe s:ReloadStatusBuffer()
   endif
 endfunction
 
@@ -2399,21 +2421,19 @@ function! s:ReloadTabStatus(...) abort
 endfunction
 
 function! fugitive#ReloadStatus(...) abort
-  call s:ExpireStatus(a:0 ? a:1 : -2)
-  if a:0 > 1 ? a:2 : s:CanAutoReloadStatus()
+  call s:ExpireStatus(a:0 ? a:1 : -1)
+  if a:0 > 1 ? a:2 : 1
     let t = reltime()
     let t:fugitive_reload_status = t
     for tabnr in exists('*settabvar') ? range(1, tabpagenr('$')) : []
       call settabvar(tabnr, 'fugitive_reload_status', t)
     endfor
     call s:ReloadTabStatus()
+    exe s:DoAutocmdChanged(a:0 ? a:1 : -1)
   else
     call s:ReloadWinStatus()
   endif
-endfunction
-
-function! s:CanAutoReloadStatus() abort
-  return get(g:, 'fugitive_autoreload_status', !has('win32'))
+  return ''
 endfunction
 
 function! fugitive#EfmDir(...) abort

plugin/fugitive.vim

@@ -10,6 +10,9 @@ let g:loaded_fugitive = 1
 
 function! FugitiveGitDir(...) abort
   if !a:0 || type(a:1) == type(0) && a:1 < 0
+    if exists('g:fugitive_event')
+      return g:fugitive_event
+    endif
     let dir = get(b:, 'git_dir', '')
     if empty(dir) && (empty(bufname('')) || &buftype =~# '^\%(nofile\|acwrite\|quickfix\|prompt\)$')
       return FugitiveExtractGitDir(getcwd())