Decouple Head() from repo object

autoload/fugitive.vim

@@ -202,6 +202,22 @@ function! fugitive#Prepare(cmd, ...) abort
   return pre . g:fugitive_git_executable . ' ' . args
 endfunction
 
+function! fugitive#Head(...) abort
+  let dir = a:0 > 1 ? a:2 : get(b:, 'git_dir', '')
+  if empty(dir) || !filereadable(dir . '/HEAD')
+    return ''
+  endif
+  let head = readfile(dir . '/HEAD')[0]
+  if head =~# '^ref: '
+    return substitute(head, '\C^ref: \%(refs/\%(heads/\|remotes/\|tags/\)\=\)\=', '', '')
+  elseif head =~# '^\x\{40\}$'
+    let len = a:0 ? a:1 : 0
+    return len < 0 ? head : len ? head[0:len-1] : ''
+  else
+    return ''
+  endif
+endfunction
+
 function! fugitive#RevParse(rev, ...) abort
   let hash = system(s:Prepare(a:0 ? a:1 : b:git_dir, 'rev-parse', '--verify', a:rev))[0:-2]
   if !v:shell_error && hash =~# '^\x\{40\}$'
@@ -408,22 +424,7 @@ function! s:Generate(rev, ...) abort
 endfunction
 
 function! s:repo_head(...) dict abort
-    if !filereadable(self.dir('HEAD'))
+  return fugitive#Head(a:0 ? a:1 : 0, self.git_dir)
-      return ''
-    endif
-    let head = readfile(self.dir('HEAD'))[0]
-
-    if head =~# '^ref: '
-      let branch = s:sub(head,'^ref: %(refs/%(heads/|remotes/|tags/)=)=','')
-    elseif head =~# '^\x\{40\}$'
-      " truncate hash to a:1 characters if we're in detached head mode
-      let len = a:0 ? a:1 : 0
-      let branch = len < 0 ? head : len ? head[0:len-1] : ''
-    else
-      return ''
-    endif
-
-    return branch
 endfunction
 
 call s:add_methods('repo',['dir','tree','bare','translate','head'])
@@ -3543,7 +3544,7 @@ function! fugitive#head(...) abort
     return ''
   endif
 
-  return fugitive#repo().head(a:0 ? a:1 : 0)
+  return fugitive#Head(a:0 ? a:1 : 0)
 endfunction
 
 " Section: Folding

plugin/fugitive.vim

@@ -85,7 +85,7 @@ function! FugitiveHead(...) abort
   if empty(dir)
     return ''
   endif
-  return fugitive#repo(dir).head(a:0 ? a:1 : 0)
+  return fugitive#Head(a:0 ? a:1 : 0, dir)
 endfunction
 
 function! FugitiveStatusline(...) abort