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 <benpeart@microsoft.com>

read-cache.c

@@ -1505,6 +1505,7 @@ 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);
 	/*
 	 * Use the multi-threaded preload_index() to refresh most of the
 	 * cache entries quickly then in the single threaded loop below,
@@ -1582,6 +1583,7 @@ int refresh_index(struct index_state *istate, unsigned int flags,
 		stop_progress(&progress);
 	}
 	trace_performance_leave("refresh index");
+	enable_fscache(0);
 	return has_errors;
 }