unpack-trees: enable fscache for sparse-checkout

When updating the skip-worktree bits in the index to align with new
values in a sparse-checkout file, Git scans the entire working
directory with lstat() calls. In a sparse-checkout, many of these
lstat() calls are for paths that do not exist.

Enable the fscache feature during this scan. Since enable_fscache()
calls nest, the disable_fscache() method decrements a counter and
would only clear the cache if that counter reaches zero.

In a local test of a repo with ~2.2 million paths, updating the index
with git read-tree -m -u HEAD with a sparse-checkout file containing
only /.gitattributes improved from 2-3 minutes to ~6 seconds.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

unpack-trees.c

@@ -1800,7 +1800,9 @@ static void mark_new_skip_worktree(struct pattern_list *pl,
 	 * 2. Widen worktree according to sparse-checkout file.
 	 * Matched entries will have skip_wt_flag cleared (i.e. "in")
 	 */
+	enable_fscache(istate->cache_nr);
 	clear_ce_flags(istate, select_flag, skip_wt_flag, pl, show_progress);
+	disable_fscache();
 }
 
 static void populate_from_existing_patterns(struct unpack_trees_options *o,