rebase: restore active bookmark after rebase --continue When a rebase has conflicts and the user uses rebase --continue, the previously active bookmark was not being made active once again. With this change that bookmark is made active again, just as if the rebase had never been interrupted. This changes the rebasestate file format, but should handle old formats correctly. Since the file is transient, this is even less of a problem. Adds a test to verify the new behavior. I manually tested continuing rebases with and without an active bookmark, and with and without being on the bookmark being rebased.