Merge branch 'lb/rebase-i-short-command-names'

Merge branch 'lb/rebase-i-short-command-names'

With a configuration variable rebase.abbreviateCommands set,
"git rebase -i" produces the todo list with a single-letter
command names.

* lb/rebase-i-short-command-names:
  sequencer.c: drop 'const' from function return type
  t3404: add test case for abbreviated commands
  rebase -i: learn to abbreviate command names
  rebase -i -x: add exec commands via the rebase--helper
  rebase -i: update functions to use a flags parameter
  rebase -i: replace reference to sha1 with oid
  rebase -i: refactor transform_todo_ids
  rebase -i: set commit to null in exec commands
  Documentation: use preferred name for the 'todo list' script
  Documentation: move rebase.* configs to new file