jj is often colocated with Git so the Git prompt also works, but
jj is always in a detached HEAD state, which is atypical for Git.
The jj prompt improves things by showing the revision ID which is
usually more useful than the commit ID.
This prompt is mostly adapted from the defaults for "jj log -r @".
Showing conflicting/empty commits seems useful.
Also perhaps bookmarks and tags, not sure.
The main problem with this prompt is that due to --ignore-working-copy,
the information may be stale. That will be rectified after every jj
command, so hopefully this doesn't cause issues.
fish_git_prompt encloses its output in brackets, however this can be changed by supplying a format string to it, i.e. `fish_git_prompt %s`.
However when using `fish_vcs_prompt` there's no way to pass on the arg to fish_git_prompt, so you need to manually remove it.
fish_hg_prompt doesn't have the same format string support as fish_git_prompt, but I suppose it could be added later if needed.
This is apparently quite slow on large svn repos (like 40 seconds
slow), and we don't have a good thing to display other than the full
file information.
So we'll have to disable it for now.
Fixes#6681.
[ci skip]
