Enable the filesystem cache (fscache) in refresh_index().

On file systems that support it, this can dramatically speed up operations
like add, commit, describe, rebase, reset, rm that would otherwise have to
lstat() every file to "re-match" the stat information in the index to that
of the file system.

On a synthetic repo with 1M files, "git reset" dropped from 52.02 seconds to
14.42 seconds for a savings of 72%.

Signed-off-by: Ben Peart <Ben.Peart@microsoft.com>

read-cache.c

@@ -1483,6 +1483,7 @@ int refresh_index(struct index_state *istate, unsigned int flags,
 	typechange_fmt = (in_porcelain ? "T\t%s\n" : "%s needs update\n");
 	added_fmt = (in_porcelain ? "A\t%s\n" : "%s needs update\n");
 	unmerged_fmt = (in_porcelain ? "U\t%s\n" : "%s: needs merge\n");
+	enable_fscache(1);
 	for (i = 0; i < istate->cache_nr; i++) {
 		struct cache_entry *ce, *new_entry;
 		int cache_errno = 0;
@@ -1547,6 +1548,7 @@ int refresh_index(struct index_state *istate, unsigned int flags,
 
 		replace_index_entry(istate, i, new_entry);
 	}
+	enable_fscache(0);
 	trace_performance_since(start, "refresh index");
 	return has_errors;
 }