worktree: honor configuration variables

The command accesses default_abbrev (defined in environment.c and is
updated via core.abbrev configuration), but never makes any call to
git_config().  The output from "worktree list" ignores the abbrev
setting for this reason.

Make a call to git_config() to read the default set of configuration
variables at the beginning of the command.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

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)