Merge branch 'jc/worktree-config' into maint

"git worktree", even though it used the default_abbrev setting that
ought to be affected by core.abbrev configuration variable, ignored
the variable setting.  The command has been taught to read the
default set of configuration variables to correct this.

* jc/worktree-config:
  worktree: honor configuration variables

builtin/worktree.c

@@ -528,6 +528,8 @@ int cmd_worktree(int ac, const char **av, const char *prefix)
 		OPT_END()
 	};
 
+	git_config(git_default_config, NULL);
+
 	if (ac < 2)
 		usage_with_options(worktree_usage, options);
 	if (!prefix)