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]
(cherry picked from commit da7b762f4a)
Adds synopses for those commands missing them.
Moves all synopsis sections to code blocks. This improves the appearance, although highlighting as
fish code may not be the ideal appearance.