Try to avoid interference by $GIT_DIR and $GIT_WORK_TREE

Resolves: https://github.com/tpope/vim-fugitive/issues/1841

autoload/fugitive.vim

@@ -428,7 +428,7 @@ function! s:UserCommandList(...) abort
     if empty(tree)
       call add(git, '--git-dir=' . FugitiveGitPath(dir))
     else
-      if !s:cpath(tree . '/.git', dir)
+      if !s:cpath(tree . '/.git', dir) || len($GIT_DIR)
         call add(git, '--git-dir=' . FugitiveGitPath(dir))
       endif
       if !s:cpath(tree, getcwd())
@@ -555,6 +555,9 @@ function! s:PrepareEnv(env, dir) abort
       let a:env['GIT_INDEX_FILE'] = FugitiveGitPath(fugitive#Find('.git/index', a:dir))
     endif
   endif
+  if len($GIT_WORK_TREE)
+    let a:env['GIT_WORK_TREE'] = '.'
+  endif
 endfunction
 
 let s:prepare_env = {
@@ -712,7 +715,7 @@ function! fugitive#PrepareJob(...) abort
   else
     let dict.cwd = FugitiveVimPath(tree)
     call extend(cmd, ['-C', FugitiveGitPath(tree)], 'keep')
-    if !s:cpath(tree . '/.git', dir)
+    if !s:cpath(tree . '/.git', dir) || len($GIT_DIR)
       call extend(cmd, ['--git-dir=' . FugitiveGitPath(dir)], 'keep')
     endif
   endif
@@ -750,7 +753,7 @@ function! s:BuildShell(dir, env, git, args) abort
     call insert(cmd, '--git-dir=' . FugitiveGitPath(a:dir))
   else
     call extend(cmd, ['-C', FugitiveGitPath(tree)], 'keep')
-    if !s:cpath(tree . '/.git', a:dir)
+    if !s:cpath(tree . '/.git', a:dir) || len($GIT_DIR)
       call extend(cmd, ['--git-dir=' . FugitiveGitPath(a:dir)], 'keep')
     endif
   endif