pullbundle: issue local branches in the right order

pullbundle: delay cache file opening Otherwise we can end-up with a too many file open at the same time.

pullbundle: improve slicing of the lower part of range The previous method could get confuse by merge and overslice. The new method is better at using sticking on power of two boundaries.

stablerange: fix slicing of arbitrary ranges Pull bundle trigger slicing from range with arbitrary initial skip. We have to adjust the current slicing to take this into account.