zsh completion: add support for convert, graphlog, purge, record extensions

zsh completion: add subrepos options for all supporting commands

zsh completion: add new option groups for options Add clone, date/user, git-style, merge tool, ignore space, log, branch/bookmarks options and use them throughout command completions.

zsh completion: add `forget` and `rollback` commands

zsh completion: complete additional topics for `help`, not commands only

Merge with stable

tests: merging bookmarked heads 45b5eb2941d0 fixed an error that occured when heads being merged were both inactive bookmarks. Add a test for that case.