tests/histedit-helpers.sh

test-histedit: add tests for dropping head changeset

I got bug report from user in this specific case. I was unable to reproduce in
test situation. Testing this situation is still valuable.

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"
}