D5291: branchmap: build the revbranchcache._namesreverse() only when required

[Yuya Nishihara]

>   > Maybe _namesreverse can be a propertycache, instead of duplicating it.
>   >  `del self.__dict__[r'...']` can be used to discard the cache.
>   
>   Hm, I am not sure if I feel good about doing `del self.__dict__[r'...']` thing.

It's pretty common in Mercurial codebase. That said, I found a better way.
`util.clearcachedproperty(self, '...')`.