unpack-trees: add 'skip_cache_tree_update' option

Add (disabled by default) option to skip the 'cache_tree_update()' at the
end of 'unpack_trees()'. In many cases, this cache tree update is redundant
because the caller of 'unpack_trees()' immediately follows it with
'prime_cache_tree()', rebuilding the entire cache tree from scratch. While
these operations aren't the most expensive part of operations like 'git
reset', the duplicate calls still create a minor unnecessary slowdown.

Introduce an option for callers to skip the 'cache_tree_update()' in
'unpack_trees()' if it is redundant (that is, if 'prime_cache_tree()' is
called afterwards). At the moment, no 'unpack_trees()' callers use the new
option; they will be updated in subsequent patches.

Signed-off-by: Victoria Dye <[email protected]>
Signed-off-by: Taylor Blau <[email protected]>

Author: vdye

unpack-trees.c

@@ -2043,7 +2043,8 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 		if (!ret) {
 			if (git_env_bool("GIT_TEST_CHECK_CACHE_TREE", 0))
 				cache_tree_verify(the_repository, &o->result);
-			if (!cache_tree_fully_valid(o->result.cache_tree))
+			if (!o->skip_cache_tree_update &&
+			    !cache_tree_fully_valid(o->result.cache_tree))
 				cache_tree_update(&o->result,
 						  WRITE_TREE_SILENT |
 						  WRITE_TREE_REPAIR);

unpack-trees.h

@@ -71,7 +71,8 @@ struct unpack_trees_options {
 		     quiet,
 		     exiting_early,
 		     show_all_errors,
-		     dry_run;
+		     dry_run,
+		     skip_cache_tree_update;
 	enum unpack_trees_reset_type reset;
 	const char *prefix;
 	int cache_bottom;