pullbundle: deal with another special case introduced by arbitrary ranges previous, we dealt with the case where the tippest subrange of the stable parent was too long. Now, we need to deal with the case where it is too small.

pullbundle: compat with Mercurial 4.7

pullbundle: add some information on the cached change This also adds a way to skip "caching" smaller range in the debugcommand (not in the extensions itself).