Merge branch 'mh/rebase-fixup'

Merge branch 'mh/rebase-fixup'

* mh/rebase-fixup:
  rebase -i: Retain user-edited commit messages after squash/fixup conflicts
  t3404: Set up more of the test repo in the "setup" step
  rebase -i: For fixup commands without squashes, do not start editor
  rebase -i: Change function make_squash_message into update_squash_message
  rebase -i: Extract function do_with_author
  rebase -i: Handle the author script all in one place in do_next
  rebase -i: Extract a function "commit_message"
  rebase -i: Simplify commit counting for generated commit messages
  rebase -i: Improve consistency of commit count in generated commit messages
  t3404: Test the commit count in commit messages generated by "rebase -i"
  rebase -i: Introduce a constant AMEND
  rebase -i: Introduce a constant AUTHOR_SCRIPT
  rebase -i: Document how temporary files are used
  rebase -i: Use symbolic constant $MSG consistently
  rebase -i: Use "test -n" instead of "test ! -z"
  rebase -i: Inline expression
  rebase -i: Remove dead code
  rebase -i: Make the condition for an "if" more transparent