util: add length argument to util.buffer()

util.buffer() either returns inbuilt buffer function or defines a new one which
slices. The inbuilt buffer() also has a length argument which is missing from
the ones we defined. This patch adds that length argument.

mercurial/util.py

@@ -238,10 +238,14 b' try:'
     buffer = buffer
 except NameError:
     if not pycompat.ispy3:
-        def buffer(sliceable, offset=0):
+        def buffer(sliceable, offset=0, length=None):
+            if length is not None:
+                return sliceable[offset:offset + length]
             return sliceable[offset:]
     else:
-        def buffer(sliceable, offset=0):
+        def buffer(sliceable, offset=0, length=None):
+            if length is not None:
+                return memoryview(sliceable)[offset:offset + length]
             return memoryview(sliceable)[offset:]
 
 closefds = pycompat.osname == 'posix'