debian packaging: maintain in fish-shell tree

Imported from https://github.com/zanchey/fish-build/

Work on #2020.

Note that this is explicitly removed from `git archive` tarballs due to
https://wiki.debian.org/UpstreamGuide#Pristine_Upstream_Source

debian/prerm

@@ -0,0 +1,46 @@
+#!/bin/sh
+# prerm script for fish
+#
+# see: dh_installdeb(1)
+
+set -e
+
+# summary of how this script can be called:
+#        * <prerm> `remove'
+#        * <old-prerm> `upgrade' <new-version>
+#        * <new-prerm> `failed-upgrade' <old-version>
+#        * <conflictor's-prerm> `remove' `in-favour' <package> <new-version>
+#        * <deconfigured's-prerm> `deconfigure' `in-favour'
+#          <package-being-installed> <version> `removing'
+#          <conflicting-package> <version>
+# for details, see http://www.debian.org/doc/debian-policy/ or
+# the debian-policy package
+
+remove_shell() {
+        if ! type remove-shell > /dev/null 2>&1; then
+                return
+        fi
+
+        remove-shell /usr/bin/fish
+}
+
+case "$1" in
+    remove|deconfigure)
+        remove_shell
+    ;;
+
+    upgrade|failed-upgrade)
+    ;;
+
+    *)
+        echo "prerm called with unknown argument \`$1'" >&2
+        exit 1
+    ;;
+esac
+
+# dh_installdeb will replace this with shell code automatically
+# generated by other debhelper scripts.
+
+#DEBHELPER#
+
+exit 0