rebase: rework extrafn handling to support multiple extrafns
This makes it possible to pass keepbranches and extrafn to rebase at
the same time, although nobody uses that functionality presently. This
is a precursor to keeping graft metadata.
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"
}