localrepo: call _poststatus hook instead of hardcoding symlink knowledge

workingctx: add _poststatus method to call _filtersuspectsymlink 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 _poststatus hook so that each context object will do the right thing depending on the situation.

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