Merge branch 'cc/replace-edit'

Merge branch 'cc/replace-edit'

"git replace" learns a new "--edit" option.

* cc/replace-edit:
  Documentation: replace: describe new --edit option
  replace: add --edit to usage string
  replace: add tests for --edit
  replace: die early if replace ref already exists
  replace: refactor checking ref validity
  replace: make sure --edit results in a different object
  replace: add --edit option
  replace: factor object resolution out of replace_object
  replace: use OPT_CMDMODE to handle modes
  replace: refactor command-mode determination