Limit apt remove ... completions to installed packages only

Added a new flag `--installed` via `argparse` to `__fish_print_packages`
which indicates that only installed packages should be listed.

TODO: Other non-debian/apt platforms should take advantage of this flag/
behavior as well.

CHANGELOG.md

@@ -80,6 +80,7 @@ This section is for changes merged to the `major` branch that are not also merge
   - `unzip`
   - `xsv`
 - Improved completions for
+  - `apt`
   - `brew`
   - `diskutil`
   - `git` (#4395, #4396, #4592)

share/completions/apt.fish

@@ -1,7 +1,8 @@
 # Completions for the `apt` command
 
 set -l all_subcmds update upgrade full-upgrade search list install show remove edit-sources purge changelog autoremove depends rdepends
-set -l pkg_subcmds install remove upgrade full-upgrade show search purge changelog policy depends rdepends
+set -l pkg_subcmds install upgrade full-upgrade show search purge changelog policy depends rdepends
+set -l installed_pkg_subcmds remove
 
 function __fish_apt_subcommand
     set subcommand $argv[1]
@@ -15,7 +16,11 @@ function __fish_apt_option
     complete -f -c apt -n "__fish_seen_subcommand_from $subcommand" $argv
 end
 
-complete -c apt -n "__fish_seen_subcommand_from $pkg_subcmds" -a '(__fish_print_packages | head -n 100)' -d 'Package'
+#using -r and not -e as string match -e is broken, this will cause problems
+#if the commandline contains special characters, but most package names do
+#not contain special characters. Can switch to -e after #4971 is fixed.
+complete -c apt -n "__fish_seen_subcommand_from $pkg_subcmds" -a '(__fish_print_packages | string match -r -- (commandline -ct) | head -n 100)' -d 'Package'
+complete -c apt -n "__fish_seen_subcommand_from $installed_pkg_subcmds" -a '(__fish_print_packages --installed | string match -r -- ".*"(commandline -ct)".*" | head -n 100)' -d 'Package'
 
 # Support flags
 complete -x -f -c apt -s h -l help     -d 'Display help'

share/functions/__fish_print_packages.fish

@@ -1,5 +1,12 @@
-
+# Use --installed to limit to installed packages only
 function __fish_print_packages
+    argparse --name=__fish_print_packages 'i/installed' -- $argv
+    or return;
+
+    set -l only_installed 1
+    if not set -q _flag_installed
+        set -e only_installed
+    end
 
     # apt-cache is much, much faster than rpm, and can do this in real
     # time. We use it if available.
@@ -13,11 +20,16 @@ function __fish_print_packages
     set -l package (_ "Package")
 
     if type -q -f apt-cache
+        if set -q only_installed
+            dpkg --get-selections | string replace -r '(\S+).*' "\$1\t$package"
+            return
+        else
             # Do not generate the cache as apparently sometimes this is slow.
             # http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=547550
             apt-cache --no-generate pkgnames (commandline -ct) 2>/dev/null | sed -e 's/$/'\t$package'/'
             return
         end
+    end
 
     # Pkg is fast on FreeBSD and provides versioning info which we want for
     # installed packages