The default hg prompt is slow on large repositories (hg status takes
2-3 seconds on mozilla-central) which is unacceptable as a default.
Mimick our git prompt: by default, only show the current branch.
If the new variable $fish_prompt_hg_show_informative_status is set,
then use the old behavior.
[ci skip]
This was written before local-exported variables did anything useful.
Passing these vars as local-exports removes the need to define the
validation function with `--no-scope-shadowing` which is quite the
hack.
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]
