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]
