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

diff --cc sequencer.c
Simple merge