narrow: fix crash when restoring backup in legacy repo Using --addremove when committing in an old repo (before we started keeping .hg/narrowspec.dirstate) results in a crash. The test case modified in this patch would crash like this: abort: $ENOENT$ The issue is that when the dirstateguard is aborted, it tries to restore the backup of .hg/narrowspec.dirstate. However, since we were in an old repo, that file did not get created when the dirstateguard was created. Note that the dirstateguard is not used unless --addremove is passed. This patch fixes the bug by making restorewcbackup() not fail if the backup doesn't exist. I also made clearwcbackup() safe, just in case. Differential Revision: https://phab.mercurial-scm.org/D5634