preload-index: avoid lstat for skip-worktree items

Teach preload-index to avoid lstat() calls for index-entries
with skip-worktree bit set.  This is a performance optimization.

During a sparse-checkout, the skip-worktree bit is set on items
that were not populated and therefore are not present in the
worktree.  The per-thread preload-index loop performs a series
of tests on each index-entry as it attempts to compare the
worktree version with the index and mark them up-to-date.
This patch short-cuts that work.

On a Windows 10 system with a very large repo (450MB index)
and various levels of sparseness, performance was improved
in the {preloadindex=true, fscache=false} case by 80% and
in the {preloadindex=true, fscache=true} case by 20% for various
commands.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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