subrepo: replace direct file APIs around "readlines" by "vfs.tryreadlines"

This patch also replaces "self._getstorehashcachepath" (building
absolute path up) by "self._getstorehashcachename" (building relative
path up), because "vfs.tryreadlines" requires relative path.

This patch makes "_readstorehashcache()" return "[]" (returned by
"vfs.tryreadlines()"), when cache file doesn't exist, even though
"_readstorehashcache()" returned '' (empty string) in such case before
this patch.

"_readstorehashcache()" is invoked only by the code path below in
"_storeclean()":

            for filehash in self._readstorehashcache(path):
                if filehash != itercache.next():
                    clean = False
                    break

In this case, "[]" and '' don't differ from each other, because both
of them cause avoiding iteration of "for loop".

mercurial/subrepo.py

@@ -572,15 +572,8 b' class hgsubrepo(abstractsubrepo):'
 
     def _readstorehashcache(self, remotepath):
         '''read the store hash cache for a given remote repository'''
-        cachefile = self._getstorehashcachepath(remotepath)
-        if not os.path.exists(cachefile):
-            return ''
-        fd = open(cachefile, 'r')
-        try:
-            pullstate = fd.readlines()
-        finally:
-            fd.close()
-        return pullstate
+        cachefile = _getstorehashcachename(remotepath)
+        return self._cachestorehashvfs.tryreadlines(cachefile, 'r')
 
     def _cachestorehash(self, remotepath):
         '''cache the current store hash