Make default hg prompt leaner

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]

doc_src/cmds/fish_hg_prompt.rst

@@ -19,7 +19,11 @@ The fish_hg_prompt function displays information about the current Mercurial rep
 
 `Mercurial <https://www.mercurial-scm.org/>`_ (``hg``) must be installed.
 
-There are numerous customization options, which can be controlled with fish variables.
+By default, only the current branch is shown because ``hg status`` can take be slow on large repository. You can enable a more informative prompt by setting the variable ``$fish_prompt_hg_show_informative_status``, for example::
+
+	set --universal fish_prompt_hg_show_informative_status
+
+If you enabled the informative status, there are numerous customization options, which can be controlled with fish variables.
 
 - ``$fish_color_hg_clean``, ``$fish_color_hg_modified`` and ``$fish_color_hg_dirty`` are colors used when the repository has the respective status.
 
@@ -52,6 +56,7 @@ A simple prompt that displays hg info::
 
     function fish_prompt
         ...
+        set -g fish_prompt_hg_show_informative_status
         printf '%s %s$' $PWD (fish_hg_prompt)
     end
 

share/functions/fish_hg_prompt.fish

@@ -36,6 +36,12 @@ function fish_hg_prompt --description 'Write out the hg prompt'
         set branch "$branch|$bookmark"
     end
 
+    if not set -q fish_prompt_hg_show_informative_status
+        set_color normal
+        echo -n " ($branch)"
+        return
+    end
+
     echo -n '|'
 
     # Disabling color and pager is always a good idea.