cat is a tool for con"cat"enating files. Reading a single file
as input to a program is considered a Useless Use Of Cat (UUOC).
It's more efficient and less roundabout to simply give file as input.

Also use $(..) instead of legacy `..`
Backtick command substitution `..` is legacy syntax with several issues.
It has a series of undefined behaviors related to quoting in POSIX.
It imposes a custom escaping mode with surprising results.
It's exceptionally hard to nest.
$(..) command substitution has none of these problems, and is therefore strongly encouraged.

Change-Id: Ia668c6323660641bbb5084ee824ae9ae7631c76f
Reviewed-on: https://gerrit.libreoffice.org/32473Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>