unpack-trees: do not mark a dirty path with SKIP_WORKTREE

If a path is dirty, removing from the working tree risks losing data. As such, we want to make sure any such path is not marked with SKIP_WORKTREE. While the current callers of this code detect this case and re-populate with a previous set of sparsity patterns, we want to allow some paths to be marked with SKIP_WORKTREE while others are left unmarked without it being considered an error. The reason this shouldn't be considered an error is that SKIP_WORKTREE has always been an advisory-only setting; merge and rebase for example were free to materialize paths and clear the SKIP_WORKTREE bit in order to accomplish their work even though they kept the SKIP_WORKTREE bit set for other paths. Leaving dirty working files in the working tree is thus a natural extension of what we have already been doing. Reviewed-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>