Merge branch 'ag/rebase-merge-allow-ff-under-abbrev-command'

"git rebase" with the merge backend did not work well when the rebase.abbreviateCommands configuration was set. * ag/rebase-merge-allow-ff-under-abbrev-command: t3432: test `--merge' with `rebase.abbreviateCommands = true', too sequencer: don't abbreviate a command if it doesn't have a short form