tests/histedit-helpers.sh

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"
}