Merge branch 'pw/rebase-r-fixes' into maint

Various bugs in "git rebase -r" have been fixed. * pw/rebase-r-fixes: rebase -r: fix merge -c with a merge strategy rebase -r: don't write .git/MERGE_MSG when fast-forwarding rebase -i: add another reword test rebase -r: make 'merge -c' behave like reword