branchmap: build the revbranchcache._namesreverse() only when required On big repositories with a lot of named branches and that also increasing over time, building of this dict can be expensive and shows up in profile. For our internal repository, this saves ~0.05 seconds. Thanks to Yuya for suggesting using util.propertycache() and util.clearcachedproperty(). Differential Revision: https://phab.mercurial-scm.org/D5291

sparse-revlog: put the native implementation of slicechunktodensity to use When possible, the C implementation of delta chain slicing will be used. providing a large boost in performance for this operation. To take a practical example of restoring manifest revision '59547c40bc4c' for a reference NetBeans repository (using sparse-revlog). The media time of the step `slice-sparse-chain` of `perfrevlogrevision` improve from 0.660 ms to 0.098 ms; The full series move delta chain slicing from 1.120 ms to 0.098 ms; Implementing _slicechunktosize into C would yield further improvements. However, the performance seems good enough for now.