sparse: refactor sparsechecksum()

This was relying on garbage collection to close the opened
file, which is a bug. Both callers simply called into self.vfs
to resolve the path. So refactor to use the vfs layer.

While we're here, rename the method to reflect it is internal
and to break anyone relying on the old behavior.

hgext/sparse.py

@@ -493,9 +493,9 b' def _wraprepo(ui, repo):'
             # resolve and can be slow.
             return self.filectx(profile, changeid=changeid).data()
 
-        def sparsechecksum(self, filepath):
-            fh = open(filepath)
-            return hashlib.sha1(fh.read()).hexdigest()
+        def _sparsechecksum(self, path):
+            data = self.vfs.read(path)
+            return hashlib.sha1(data).hexdigest()
 
         def _sparsesignature(self, includetemp=True):
             """Returns the signature string representing the contents of the
@@ -511,8 +511,7 b' def _wraprepo(ui, repo):'
             if signature is None or (includetemp and tempsignature is None):
                 signature = 0
                 try:
-                    sparsepath = self.vfs.join('sparse')
-                    signature = self.sparsechecksum(sparsepath)
+                    signature = self._sparsechecksum('sparse')
                 except (OSError, IOError):
                     pass
                 signaturecache['signature'] = signature
@@ -520,8 +519,7 b' def _wraprepo(ui, repo):'
                 tempsignature = 0
                 if includetemp:
                     try:
-                        tempsparsepath = self.vfs.join('tempsparse')
-                        tempsignature = self.sparsechecksum(tempsparsepath)
+                        tempsignature = self._sparsechecksum('tempsparse')
                     except (OSError, IOError):
                         pass
                     signaturecache['tempsignature'] = tempsignature