rebase: improve error message for empty --source set
Before, it just said 'nothing to rebase' in this case. Now, it aborts
mentioning the reason: 'empty "source" revision set'.
Specifying revisions that cannot be rebased is a 'soft' error, but specifying
an empty set deserves an abort that explains exactly what the problem is.
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"
}