Merge branch 'cc/untracked'

Update the untracked cache subsystem and change its primary UI from
"git update-index" to "git config".

* cc/untracked:
  t7063: add tests for core.untrackedCache
  test-dump-untracked-cache: don't modify the untracked cache
  config: add core.untrackedCache
  dir: simplify untracked cache "ident" field
  dir: add remove_untracked_cache()
  dir: add {new,add}_untracked_cache()
  update-index: move 'uc' var declaration
  update-index: add untracked cache notifications
  update-index: add --test-untracked-cache
  update-index: use enum for untracked cache options
  dir: free untracked cache when removing it

environment.c

@@ -87,6 +87,13 @@ int auto_comment_line_char;
 /* Parallel index stat data preload? */
 int core_preload_index = 1;
 
+/*
+ * This is a hack for test programs like test-dump-untracked-cache to
+ * ensure that they do not modify the untracked cache when reading it.
+ * Do not use it otherwise!
+ */
+int ignore_untracked_cache_config;
+
 /* This is set by setup_git_dir_gently() and/or git_default_config() */
 char *git_work_tree_cfg;
 static char *work_tree;