commit: write cache-tree data when writing index anyway

In prepare_index(), we refresh the index, and then write it to disk if
this changed the index data.  After running hooks we re-read the index
and compute the root tree sha1 with the cache-tree machinery.

This gives us a mostly free opportunity to write up-to-date cache-tree
data: we can compute it in prepare_index() immediately before writing
the index to disk.

If we do this, we were going to write the index anyway, and the later
cache-tree update has no further work to do.  If we don't do it, we
don't do any extra work, though we still don't have have cache-tree
data after the commit.

The only case that suffers badly is when the pre-commit hook changes
many trees in the index.  I'm writing this off as highly unusual.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -394,6 +394,7 @@ static char *prepare_index(int argc, const char **argv, const char *prefix,
 		fd = hold_locked_index(&index_lock, 1);
 		add_files_to_cache(also ? prefix : NULL, pathspec, 0);
 		refresh_cache_or_die(refresh_flags);
+		update_main_cache_tree(1);
 		if (write_cache(fd, active_cache, active_nr) ||
 		    close_lock_file(&index_lock))
 			die(_("unable to write new_index file"));
@@ -414,6 +415,7 @@ static char *prepare_index(int argc, const char **argv, const char *prefix,
 		fd = hold_locked_index(&index_lock, 1);
 		refresh_cache_or_die(refresh_flags);
 		if (active_cache_changed) {
+			update_main_cache_tree(1);
 			if (write_cache(fd, active_cache, active_nr) ||
 			    commit_locked_index(&index_lock))
 				die(_("unable to write new_index file"));

t/t0090-cache-tree.sh

@@ -70,7 +70,7 @@ test_expect_success 'test-scrap-cache-tree works' '
 	test_no_cache_tree
 '
 
-test_expect_failure 'second commit has cache-tree' '
+test_expect_success 'second commit has cache-tree' '
 	test_commit bar &&
 	test_shallow_cache_tree
 '