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