work around unset SAGE_ROOT in Debian Sage package

work around unset SAGE_ROOT in Debian Sage package

Bug-Debian: https://bugs.debian.org/972346
Forwarded: https://bugs.kde.org/show_bug.cgi?id=408176#c19
Last-Update: 2021-01-17

In this shell script, '$1' refers to the Sage binary that is to
be used. The script is needlessly complex to accommodate for old
versions of Sage, and perhaps the author is unaware of Sage's
new command-line arguments.

Debian's `sage --root` is an empty string, so Cantor fails to
start it. This patch is more succinct by starting the Sage shell
which sets all variables appropriately, then starting
'sage-ipython' as desired. This way, as opposed to having
'#!/usr/bin/sage --sh' as the shebang, still permits usage of
local Sage copies.

Gbp-Pq: Name work-around-Sage-no-root.diff