vfs: ignore EPERM at os.utime, which avoids ambiguity at renaming (issue5418)

According to POSIX specification, just having group write access to a
file causes EPERM at invocation of os.utime() with an explicit time
information (e.g. working on the repository shared by group access
permission).

To ignore EPERM at renaming in such case, this patch makes
vfs.rename() use filestat.avoidambig() introduced by previous patch.

mercurial/scmutil.py

@@ -390,8 +390,7 b' class abstractvfs(object):'
             newstat = util.filestat(dstpath)
             if newstat.isambig(oldstat):
                 # stat of renamed file is ambiguous to original one
-                advanced = (oldstat.stat.st_mtime + 1) & 0x7fffffff
-                os.utime(dstpath, (advanced, advanced))
+                newstat.avoidambig(dstpath, oldstat)
             return ret
         return util.rename(self.join(src), dstpath)