Fix return value for `rebase_rewritten` in "rebase-apply" mode
Fix return value for `rebase_rewritten` in "rebase-apply" mode

`rebase_rewritten` returns a map from old to new sha.  Returning a
list has been introduced in becc7ab6 (Feature: Preserve merges rebase).

This has been pointed out in the review of #534 by @asfaltboy but not
resolved.

divmain/GitSavvy,divmain/GitSavvy,divmain/GitSavvy