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.
fixbundle() { grep -v 'saving bundle' | grep -v 'saved backup' | \ grep -v added | grep -v adding | \ grep -v "unable to find 'e' for patching" | \ grep -v "e: No such file or directory"}