localrepo: add isfilecached to check filecache-ed property is already cached

isfilecached() encapsulates internal implementation of filecache-ed
property.

"name in repo.unfiltered().__dict__" or so can't be used for this
purpose, because corresponded entry in __dict__ might be discarded by
repo.invalidate(), repo.invalidatedirstate() or so (fsmonitor does so,
for example).

This patch makes isfilecached() return not only whether filecache-ed
property is already cached, but also already cached value (or None),
in order to avoid subsequent access to cached object via "repo.NAME",
which prevents main Mercurial procedure after reposetup() from
validating cache.

mercurial/localrepo.py

@@ -104,6 +104,16 b' class storecache(_basefilecache):'
     def join(self, obj, fname):
         return obj.sjoin(fname)
 
+def isfilecached(repo, name):
+    """check if a repo has already cached "name" filecache-ed property
+
+    This returns (cachedobj-or-None, iscached) tuple.
+    """
+    cacheentry = repo.unfiltered()._filecache.get(name, None)
+    if not cacheentry:
+        return None, False
+    return cacheentry.obj, True
+
 class unfilteredpropertycache(util.propertycache):
     """propertycache that apply to unfiltered repo only"""