contrib: Silence git-describe when looking for tag

Otherwise, it prints a rather disturbing message to stderr:

    fatal: no tag exactly matches '<hash>'

contrib/gitian-descriptors/assign_DISTNAME

@@ -4,7 +4,7 @@
 #
 # A helper script to be sourced into the gitian descriptors
 
-if RECENT_TAG="$(git describe --exact-match HEAD)"; then
+if RECENT_TAG="$(git describe --exact-match HEAD 2> /dev/null)"; then
     VERSION="${RECENT_TAG#v}"
 else
     VERSION="$(git rev-parse --short=12 HEAD)"