worktree remove: clarify error message on dirty worktree

worktree remove: clarify error message on dirty worktree

To avoid data loss, 'git worktree remove' refuses to delete a worktree
if it's dirty or contains untracked files.  However, the error message
only mentions that the worktree "is dirty", even if the worktree in
question is in fact clean, but contains untracked files:

  $ git worktree add test-worktree
  Preparing worktree (new branch 'test-worktree')
  HEAD is now at aa53e60 Initial
  $ >test-worktree/untracked-file
  $ git worktree remove test-worktree/
  fatal: 'test-worktree/' is dirty, use --force to delete it
  $ git -C test-worktree/ diff
  $ git -C test-worktree/ diff --cached
  $ # Huh?  Where are those dirty files?!

Clarify this error message to say that the worktree "contains modified
or untracked files".

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>