Trusting fugitive instead of tracking paths manually

The initial reason in #237 is not valid anymore, as vim-gitgutter
functions as expected for editing files that are not part of a repo,
whether they are:
- in a different repo
- in a parent repo (cwd being a submodule)
- outside of the repo

Furthermore, removing this check allows to show relevant info for
specific fugitive file names that are fugitive://..../sha1//...
which are hard to parse manually, especially in complicated situations
such as submodules.

autoload/airline/extensions/branch.vim

@@ -247,9 +247,6 @@ function! airline#extensions#branch#head()
     endif
   endif
 
-  if has_key(heads, 'git') && !s:check_in_path()
-    let b:airline_head = ''
-  endif
   let minwidth = empty(get(b:, 'airline_hunks', '')) ? 14 : 7
   let b:airline_head = airline#util#shorten(b:airline_head, 120, minwidth)
   return b:airline_head
@@ -264,35 +261,6 @@ function! airline#extensions#branch#get_head()
         \ : printf('%s%s', empty(symbol) ? '' : symbol.(g:airline_symbols.space), head)
 endfunction
 
-function! s:check_in_path()
-  if !exists('b:airline_file_in_root')
-    let root = get(b:, 'git_dir', get(b:, 'mercurial_dir', ''))
-    let bufferpath = resolve(fnamemodify(expand('%'), ':p'))
-
-    if !filereadable(root) "not a file
-      " if .git is a directory, it's the old submodule format
-      if match(root, '\.git$') >= 0
-        let root = expand(fnamemodify(root, ':h'))
-      else
-        " else it's the newer format, and we need to guesstimate
-        " 1) check for worktrees
-        if match(root, 'worktrees') > -1
-          " worktree can be anywhere, so simply assume true here
-          return 1
-        endif
-        " 2) check for submodules
-        let pattern = '\.git[\\/]\(modules\)[\\/]'
-        if match(root, pattern) >= 0
-          let root = substitute(root, pattern, '', '')
-        endif
-      endif
-    endif
-
-    let b:airline_file_in_root = stridx(bufferpath, root) > -1
-  endif
-  return b:airline_file_in_root
-endfunction
-
 function! s:reset_untracked_cache(shellcmdpost)
   " shellcmdpost - whether function was called as a result of ShellCmdPost hook
   if !g:airline#init#vim_async && !has('nvim')
@@ -319,7 +287,6 @@ endfunction
 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 ShellCmdPost,CmdwinLeave * unlet! b:airline_head b:airline_do_mq_check
   autocmd User AirlineBeforeRefresh unlet! b:airline_head b:airline_do_mq_check
   autocmd BufWritePost * call s:reset_untracked_cache(0)