graft: use a real conflict for the tests One of the graft tests grafts a changeset that changes a file's content from 'a' to 'b' onto a branch that has changed the file's content from 'a', via 'b', and then back to 'a' again. To prepare for not considering this a file in need of merging, let's use 'c' as the file's new content to make sure it has to be considered conflicting. There's a second similar case further down where an ancestor is grafted. Make sure that is also considered a conflict.

add: use lexists so that broken symbolic links are added This restores the add behavior prior to d8cdd46f426d and matches the behavior of addremove.

rebase: avoid redundant repo[rev].rev() - just keep working in rev space

rebase: always store rev in state, also when collapsing

rebase: clarify naming of variables holding node hashes - don't call them rev

bookmarks: factor out bookmark file opening for easier extensibility