remove pointless os.path.join calls when opening files in .hg/cache

Windows deals just fine with '/' in paths and Mercurial on Windows
already does file accesses elsewhere with mixed \ and / in file
paths anyway.

This patch also makes the static-http repo case use proper URLs of the
form

  http://example.com/repo/.hg/cache/branchheads
  http://example.com/repo/.hg/cache/tags

instead of the entirely pointless

  http://example.com/repo/.hg/cache%5Cbranchheads
  http://example.com/repo/.hg/cache%5tags

(as introduced by 5ccdca7df211)

mercurial/localrepo.py

@@ -439,7 +439,7 b' class localrepository(repo.repository):'
     def _readbranchcache(self):
         partial = {}
         try:
-            f = self.opener(os.path.join("cache", "branchheads"))
+            f = self.opener("cache/branchheads")
             lines = f.read().split('\n')
             f.close()
         except (IOError, OSError):
@@ -467,8 +467,7 b' class localrepository(repo.repository):'
 
     def _writebranchcache(self, branches, tip, tiprev):
         try:
-            f = self.opener(os.path.join("cache", "branchheads"), "w",
-                            atomictemp=True)
+            f = self.opener("cache/branchheads", "w", atomictemp=True)
             f.write("%s %s\n" % (hex(tip), tiprev))
             for label, nodes in branches.iteritems():
                 for node in nodes:

mercurial/tags.py

@@ -152,7 +152,7 b' def _readtagcache(ui, repo):'
     set, caller is responsible for reading tag info from each head.'''
 
     try:
-        cachefile = repo.opener(os.path.join('cache', 'tags'), 'r')
+        cachefile = repo.opener('cache/tags', 'r')
         # force reading the file for static-http
         cachelines = iter(cachefile)
     except IOError:
@@ -249,8 +249,7 b' def _readtagcache(ui, repo):'
 def _writetagcache(ui, repo, heads, tagfnode, cachetags):
 
     try:
-        cachefile = repo.opener(os.path.join('cache', 'tags'), 'w',
-                                atomictemp=True)
+        cachefile = repo.opener('cache/tags', 'w', atomictemp=True)
     except (OSError, IOError):
         return