commit: use context manager with dirstateguard When I wrote 5ac845ca059a (commit: don't let failed commit with --addremove update dirstate (issue5645), 2017-07-31), Durham's 609606d21765 (rebase: use one dirstateguard for when using rebase.singletransaction, 2017-07-20) had not yet landed, so I had to write it in the old-fashioned way. Now that Durham's patch is in, we can simplify by using a context manager. Differential Revision: https://phab.mercurial-scm.org/D406