rel-builds: Specify core.abbrev for git-rev-parse

Chose 12 because the kernel uses it:

  https://public-inbox.org/git/CA+55aFy0_pwtFOYS1Tmnxipw9ZkRNCQHmoYyegO00pjMiZQfbg@mail.gmail.com/raw

And also because it's a nice number.

contrib/gitian-descriptors/assign_DISTNAME

@@ -7,6 +7,6 @@
 if RECENT_TAG="$(git describe --exact-match HEAD)"; then
     VERSION="${RECENT_TAG#v}"
 else
-    VERSION="$(git rev-parse --short HEAD)"
+    VERSION="$(git rev-parse --short=12 HEAD)"
 fi
 DISTNAME="bitcoin-${VERSION}"