Add a "prompt_login" helper function

This prints a description of the "host". Currently that's

`(chroot:debianchroot) $USER@$hostname`

with the chroot part when needed.

This also switches the default and terlar prompts to use it, the other
prompts have slightly different coloring or logic here.

doc_src/cmds/prompt_login.rst

@@ -0,0 +1,26 @@
+.. _cmd-prompt_login:
+
+prompt_login - describe the login suitable for prompt
+=====================================================
+
+Synopsis
+--------
+
+::
+
+    function fish_prompt
+        echo -n (prompt_login) (prompt_pwd) '$ '
+    end
+
+Description
+-----------
+
+``prompt_login`` is a function to describe the current login. It will show the user, the host and also whether the shell is running in a chroot (currently debian's debian_chroot is supported).
+
+Examples
+--------
+
+::
+
+    >_ prompt_login
+    root@bananablaster

share/functions/fish_prompt.fish

@@ -16,12 +16,6 @@ function fish_prompt --description 'Write out the prompt'
         set suffix '#'
     end
 
-    # If we're running via SSH, change the host color.
-    set -l color_host $fish_color_host
-    if set -q SSH_TTY
-        set color_host $fish_color_host_remote
-    end
-
     # Write pipestatus
     # If the status was carried over (e.g. after `set`), don't bold it.
     set -l bold_flag --bold
@@ -34,5 +28,5 @@ function fish_prompt --description 'Write out the prompt'
     set -l statusb_color (set_color $bold_flag $fish_color_status)
     set -l prompt_status (__fish_print_pipestatus "[" "]" "|" "$status_color" "$statusb_color" $last_pipestatus)
 
-    echo -n -s (set_color $fish_color_user) "$USER" $normal @ (set_color $color_host) (prompt_hostname) $normal ' ' (set_color $color_cwd) (prompt_pwd) $normal (fish_vcs_prompt) $normal " "$prompt_status $suffix " "
+    echo -n -s (prompt_login)' ' (set_color $color_cwd) (prompt_pwd) $normal (fish_vcs_prompt) $normal " "$prompt_status $suffix " "
 end

share/functions/prompt_login.fish

@@ -0,0 +1,28 @@
+function prompt_login --description "Print a description of the user and host suitable for the prompt"
+    if not set -q __fish_machine
+        set -g __fish_machine
+        set -l debian_chroot $debian_chroot
+
+        if  test -r /etc/debian_chroot
+            set debian_chroot (cat /etc/debian_chroot)
+        end
+
+        if set -q debian_chroot[1]
+            and test -n "$debian_chroot"
+            set -g __fish_machine "(chroot:$debian_chroot)"
+        end
+    end
+
+    # Prepend the chroot environment if present
+    if set -q __fish_machine[1]
+        echo -n -s (set_color yellow) "$__fish_machine" (set_color normal) ' '
+    end
+
+    # If we're running via SSH, change the host color.
+    set -l color_host $fish_color_host
+    if set -q SSH_TTY; and set -q fish_color_host_remote
+        set color_host $fish_color_host_remote
+    end
+
+    echo -n -s (set_color $fish_color_user) "$USER" (set_color normal) @ (set_color $color_host) (prompt_hostname) (set_color normal)
+end

share/tools/web_config/sample_prompts/default.fish

@@ -16,12 +16,6 @@ function fish_prompt --description 'Write out the prompt'
         set suffix '#'
     end
 
-    # If we're running via SSH, change the host color.
-    set -l color_host $fish_color_host
-    if set -q SSH_TTY
-        set color_host $fish_color_host_remote
-    end
-
     # Write pipestatus
     # If the status was carried over (e.g. after `set`), don't bold it.
     set -l bold_flag --bold
@@ -34,5 +28,5 @@ function fish_prompt --description 'Write out the prompt'
     set -l statusb_color (set_color $bold_flag $fish_color_status)
     set -l prompt_status (__fish_print_pipestatus "[" "]" "|" "$status_color" "$statusb_color" $last_pipestatus)
 
-    echo -n -s (set_color $fish_color_user) "$USER" $normal @ (set_color $color_host) (prompt_hostname) $normal ' ' (set_color $color_cwd) (prompt_pwd) $normal (fish_vcs_prompt) $normal " "$prompt_status $suffix " "
+    echo -n -s (prompt_login)' ' (set_color $color_cwd) (prompt_pwd) $normal (fish_vcs_prompt) $normal " "$prompt_status $suffix " "
 end

share/tools/web_config/sample_prompts/terlar.fish

@@ -4,17 +4,7 @@
 function fish_prompt --description 'Write out the prompt'
     set -l last_status $status
 
-    # User
+    prompt_login
-    set_color $fish_color_user
-    echo -n $USER
-    set_color normal
-
-    echo -n '@'
-
-    # Host
-    set_color $fish_color_host
-    echo -n (prompt_hostname)
-    set_color normal
 
     echo -n ':'