localrepo: call _prestatus hook instead of hardcoding dirstate knowledge

workingctx: add _prestatus method to call _dirstatestatus With this patch, we are one step closer to removing 'if ... else' logic in localrepo.status for testing if the context is the working directory or not. Future patches will replace those blocks of code with a call to the context's _prestatus hook so that each context object will do the right thing depending on the situation.

context: add a no-op _prestatus method This patch adds a private _prestatus method so that certain contexts, such as workingctx, can add custom pre-processing to status.