Support .git-file repositories

This supports the new submodule structure and closes #139.

plugin/fugitive.vim

@@ -121,8 +121,14 @@ function! s:extract_git_dir(path) abort
   let ofn = ""
   let nfn = fn
   while fn !=# ofn
-    if s:is_git_dir(s:sub(fn,'[\/]$','') . '/.git')
+    let embedded = s:sub(fn, '[\/]$', '') . '/.git'
+    if s:is_git_dir(embedded)
       return s:sub(simplify(fnamemodify(fn . '/.git',':p')),'\W$','')
+    elseif filereadable(embedded)
+      let line = readfile(embedded,1)[0]
+      if line =~# '^gitdir: '
+        return line[8:-1]
+      endif
     elseif s:is_git_dir(fn)
       return s:sub(simplify(fnamemodify(fn,':p')),'\W$','')
     endif
@@ -196,15 +202,28 @@ function! s:repo_dir(...) dict abort
 endfunction
 
 function! s:repo_tree(...) dict abort
-  if !self.bare()
+  if self.dir() =~# '/\.git$'
-    let dir = fnamemodify(self.git_dir,':h')
+    let dir = self.dir()[0:-6]
-    return join([dir]+a:000,'/')
+  else
+    let config = readfile(self.dir('config'),10)
+    call filter(config,'v:val =~# "^\\s*worktree *="')
+    if len(config) == 1
+      let dir = matchstr(config[0], '= *\zs.*')
+    else
+      call s:throw('no work tree')
+    endif
   endif
-  call s:throw('no work tree')
+  return join([dir]+a:000,'/')
 endfunction
 
 function! s:repo_bare() dict abort
-  return self.dir() !~# '/\.git$'
+  if self.dir() =~# '/\.git$'
+    return 0
+  else
+    let config = readfile(self.dir('config'),10)
+    call filter(config,'v:val =~# "^\\s*worktree *="')
+    return (len(config) != 1)
+  endtry
 endfunction
 
 function! s:repo_translate(spec) dict abort