vfs: add "readlines" and "tryreadlines"

This patch allows "readlines" and "tryreadlines" to take "mode"
argument, because "subrepo" requires to read files not in "rb"
(binary, default for vfs) but in "r" (text) mode in subsequent patch.

mercurial/scmutil.py

@@ -188,6 +188,15 @@ class abstractvfs(object):
                 raise
         return ""
 
+    def tryreadlines(self, path, mode='rb'):
+        '''gracefully return an empty array for missing files'''
+        try:
+            return self.readlines(path, mode=mode)
+        except IOError, inst:
+            if inst.errno != errno.ENOENT:
+                raise
+        return []
+
     def open(self, path, mode="r", text=False, atomictemp=False):
         self.open = self.__call__
         return self.__call__(path, mode, text, atomictemp)
@@ -199,6 +208,13 @@ class abstractvfs(object):
         finally:
             fp.close()
 
+    def readlines(self, path, mode='rb'):
+        fp = self(path, mode=mode)
+        try:
+            return fp.readlines()
+        finally:
+            fp.close()
+
     def write(self, path, data):
         fp = self(path, 'wb')
         try: