Use filename in clean_smudge_filter_applies() function

And move function to utility.

Since we change directory to the file's directory we can just use its
name rather than it path from the repo root.

autoload/gitgutter/hunk.vim

@@ -295,7 +295,7 @@ endfunction
 function! s:stage(hunk_diff)
   let bufnr = bufnr('')
 
-  if s:clean_smudge_filter_applies(bufnr)
+  if gitgutter#utility#clean_smudge_filter_applies(bufnr)
     let choice = input('File uses clean/smudge filter. Stage entire file (y/n)? ')
     normal! :<ESC>
     if choice =~ 'y'
@@ -654,17 +654,3 @@ function gitgutter#hunk#is_preview_window_open()
   endif
   return 0
 endfunction
-
-
-function! s:clean_smudge_filter_applies(bufnr)
-  let filtered = gitgutter#utility#getbufvar(a:bufnr, 'filter', -1)
-  if filtered == -1
-    let path = gitgutter#utility#repo_path(a:bufnr, 1)
-    let out = gitgutter#utility#system(
-          \ gitgutter#utility#cd_cmd(a:bufnr, g:gitgutter_git_executable.' '.g:gitgutter_git_args.' check-attr filter -- '.path)
-          \ )
-    let filtered = out !~ 'unspecified'
-    call gitgutter#utility#setbufvar(a:bufnr, 'filter', filtered)
-  endif
-  return filtered
-endfunction

autoload/gitgutter/utility.vim

@@ -178,6 +178,21 @@ function! gitgutter#utility#set_repo_path(bufnr, continuation) abort
 endfunction
 
 
+function! gitgutter#utility#clean_smudge_filter_applies(bufnr)
+  let filtered = gitgutter#utility#getbufvar(a:bufnr, 'filter', -1)
+  if filtered == -1
+    let cmd = gitgutter#utility#cd_cmd(a:bufnr,
+          \ g:gitgutter_git_executable.' '.g:gitgutter_git_args.
+          \ ' check-attr filter -- '.
+          \ gitgutter#utility#shellescape(gitgutter#utility#filename(a:bufnr)))
+    let out = gitgutter#utility#system(cmd)
+    let filtered = out !~ 'unspecified'
+    call gitgutter#utility#setbufvar(a:bufnr, 'filter', filtered)
+  endif
+  return filtered
+endfunction
+
+
 function! gitgutter#utility#cd_cmd(bufnr, cmd) abort
   let cd = s:unc_path(a:bufnr) ? 'pushd' : (gitgutter#utility#windows() && s:dos_shell() ? 'cd /d' : 'cd')
   return cd.' '.s:dir(a:bufnr).' && '.a:cmd