pullbundle: add data about the cached bundle size and hit

pullbundle: add a command to generate cache hit statistic This new command should help checking that the stable range produce reusable bundle.

pullbundle: point out the output flooding issue from core Right now, we don't have a reliable progress report when using pullbundle.

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.

pullbundle: attempt to save stablerange cache after each computation Since we do not serve the full repository, we use more stablerange starting from a random point. These are probably not loaded yet and should be cached. This is best effort caching, if we can't save them we won't.