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

mercurial/narrowspec.py

@@ -190,12 +190,14 b' def savewcbackup(repo, backupname):'
 def restorewcbackup(repo, backupname):
     if repository.NARROW_REQUIREMENT not in repo.requirements:
         return
+    # It may not exist in old repos
+    if repo.vfs.exists(backupname):
     util.rename(repo.vfs.join(backupname), repo.vfs.join(DIRSTATE_FILENAME))
 
 def clearwcbackup(repo, backupname):
     if repository.NARROW_REQUIREMENT not in repo.requirements:
         return
-    repo.vfs.unlink(backupname)
+    repo.vfs.tryunlink(backupname)
 
 def restrictpatterns(req_includes, req_excludes, repo_includes, repo_excludes):
     r""" Restricts the patterns according to repo settings,

tests/test-narrow-share.t

@@ -166,7 +166,7 b' Dirstate should be left alone when upgra'
   R d7/f
 Make it look like a repo from before narrow+share was supported
   $ rm .hg/narrowspec.dirstate
-  $ hg st
+  $ hg ci -Am test
   abort: working copy's narrowspec is stale
   (run 'hg tracked --update-working-copy')
   [255]