convert: clear the dirstate before a conversion, invalidate it afterwards
Clearing it before the conversion protects us from whatever data were
there (file copies in particular).
Invalidating it after the conversion avoids writing a possibly
inconsistent dirstate to disk.
adding b
rev offset length base linkrev nodeid p1 p2
0 0 3 0 0 1e88685f5dde 000000000000 000000000000