update-index/diff-index: use core.preloadindex to improve performance

'update-index --refresh' and 'diff-index' (without --cached) don't honor
the core.preloadindex setting yet. Porcelain commands using these (such as
git [svn] rebase) suffer from this, especially on Windows.

Use read_cache_preload to improve performance.

Additionally, in builtin/diff.c, don't preload index status if we don't
access the working copy (--cached).

Results with msysgit on WebKit repo (2GB in 200k files):

                | update-index | diff-index | rebase
----------------+--------------+------------+---------
msysgit-v1.8.0  |       9.157s |    10.536s | 42.791s
+ preloadindex  |       9.157s |    10.536s | 28.725s
+ this patch    |       2.329s |     2.752s | 15.152s
+ fscache [1]   |       0.731s |     1.171s |  8.877s

[1] https://github.com/kblees/git/tree/kb/fscache-v3

Thanks-to: Albert Krawczyk <pro-logic@optusnet.com.au>
Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Jeff King <peff@peff.net>

builtin/diff-index.c

@@ -41,9 +41,13 @@ int cmd_diff_index(int argc, const char **argv, const char *prefix)
 	if (rev.pending.nr != 1 ||
 	    rev.max_count != -1 || rev.min_age != -1 || rev.max_age != -1)
 		usage(diff_cache_usage);
-	if (!cached)
+	if (!cached) {
 		setup_work_tree();
-	if (read_cache() < 0) {
+		if (read_cache_preload(rev.diffopt.pathspec.raw) < 0) {
+			perror("read_cache_preload");
+			return -1;
+		}
+	} else if (read_cache() < 0) {
 		perror("read_cache");
 		return -1;
 	}

builtin/diff.c

@@ -130,8 +130,6 @@ static int builtin_diff_index(struct rev_info *revs,
 			usage(builtin_diff_usage);
 		argv++; argc--;
 	}
-	if (!cached)
-		setup_work_tree();
 	/*
 	 * Make sure there is one revision (i.e. pending object),
 	 * and there is no revision filtering parameters.
@@ -140,8 +138,14 @@ static int builtin_diff_index(struct rev_info *revs,
 	    revs->max_count != -1 || revs->min_age != -1 ||
 	    revs->max_age != -1)
 		usage(builtin_diff_usage);
-	if (read_cache_preload(revs->diffopt.pathspec.raw) < 0) {
-		perror("read_cache_preload");
+	if (!cached) {
+		setup_work_tree();
+		if (read_cache_preload(revs->diffopt.pathspec.raw) < 0) {
+			perror("read_cache_preload");
+			return -1;
+		}
+	} else if (read_cache() < 0) {
+		perror("read_cache");
 		return -1;
 	}
 	return run_diff_index(revs, cached);

builtin/update-index.c

@@ -593,6 +593,7 @@ struct refresh_params {
 static int refresh(struct refresh_params *o, unsigned int flag)
 {
 	setup_work_tree();
+	read_cache_preload(NULL);
 	*o->has_errors |= refresh_cache(o->flags | flag);
 	return 0;
 }