opener: introduce tryread helper

This makes it easier to follow the common pattern "read a file or give
an empty string if it's missing".

mercurial/scmutil.py

@@ -159,6 +159,15 b' class abstractopener(object):'
         '''Prevent instantiation; don't call this from subclasses.'''
         raise NotImplementedError('attempted instantiating ' + str(type(self)))
 
+    def tryread(self, path):
+        'gracefully return an empty string for missing files'
+        try:
+            return self.read(path)
+        except IOError, inst:
+            if inst.errno != errno.ENOENT:
+                raise
+        return ""
+
     def read(self, path):
         fp = self(path, 'rb')
         try: