dirstate: speed up read and write read: - single call to len(st) - fewer assignments for position tracking - don't split apart tuple from unpack - use a literal for the unpack spec write: - localize variables and functions - avoid copied function call - use % for string concatenation

adding b
   rev    offset  length   base linkrev nodeid       p1           p2
     0         0       3      0       0 1e88685f5dde 000000000000 000000000000