narrow: drop checkambig=True when restoring backup

IIUC, checkambig is about updating timestamps of the file while
renaming. That's important for the dirstate, but we never check the
timestamp of the narrowspec file. We can therefore avoid checking
passing checkambig=True.

Differential Revision: https://phab.mercurial-scm.org/D4098

mercurial/narrowspec.py

@@ -137,7 +137,7 b' def savebackup(repo, backupname):'
 def restorebackup(repo, backupname):
     if repository.NARROW_REQUIREMENT not in repo.requirements:
         return
-    repo.vfs.rename(backupname, FILENAME, checkambig=True)
+    repo.vfs.rename(backupname, FILENAME)
 
 def clearbackup(repo, backupname):
     if repository.NARROW_REQUIREMENT not in repo.requirements: