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

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>