Since commit 5067d313 the SHELL variable
is no longer set to bash in Makefile.in, so $PIPESTATUS is empty and the
fetch command fails due to syntax error (as reported by Olivier Hallot).

Since i have no idea how to implement the equivalent of $PIPESTATUS for
shells that lack it, just invoke bash explicitly here.

Change-Id: I7e27377e54c7e19175abcbec0beefca31a55a2bd
Reviewed-on: https://gerrit.libreoffice.org/19164Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>