rebase: write series file without removed mq patches
Rebase will remove empty changesets and will also completely remove the mq
patch file for rebased empty patches.
Starting with
b28004513977 (1.9) it would preserve guards by writing the old
series file back. That would however also reintroduce removed patch files in
the series file and the inconsistency would make qpop + qpush fail.
This patch backs out most of
b28004513977 and makes sure guards are preserved
without reintroducing removed patches.