Using the automation-friendly "fossil branch current" feature added in
Fossil 2.7 instead of ad hoc parsing of human-readable "fossil branch"
output. Not only does this fix a stray space in the output, it's more
robust against changes in command output in general.
Closes#11138
Instead of blindly tacking `$fossil_prompt_info` onto the end of the
prompt string and making assumptions about whether `$PROMPT` or
`$RPROMPT` are in use here, assume the user is using a theme that puts
`$git_prompt_info` in there somewhere and inject our info immediately
afterward. The theme creator decided that's a good place for Git prompt
info, so that's a good place for Fossil prompt info, too.
This also replaces some calls out to `grep` with internal Zsh string
manipulation for less overhead.