Merge branch 'pw/rebase-i-merge-segv-fix'

"git rebase -i", when a 'merge <branch>' insn in its todo list fails, segfaulted, which has been (minimally) corrected. * pw/rebase-i-merge-segv-fix: rebase -i: fix SIGSEGV when 'merge <branch>' fails t3430: add conflicting commit