Vim: Return the Git root directory / folder (Git top level).

vim_git_root_directory.vim

" Language: Vim script
" Author: James Cherti 
" URL: https://gist.github.com/jamescherti/c36c975aad4802a0e47a4a8274d55684
" License: MIT
" Description: Return the Git root directory / folder (Git top level).
"              let toplevel = GitRootDir()
"              let toplevel = GitRootDir("/path/to/dir")

function! GitRootDir(...) abort
  if len(a:000) > 0
    let l:dir = a:000[0]
  else
    let l:dir = getcwd()
  endif

  let l:gitdir = systemlist('git -C ' . shellescape(l:dir) .
        \ ' rev-parse --show-toplevel')
  if len(l:gitdir) > 0 && isdirectory(l:gitdir[0])
    return l:gitdir[0]
  endif

  return ''
endfunction