configure: embed short hash for the git revision.

configure

@@ -193,7 +193,7 @@ echo "CPPFLAGS +=	-DXBPS_VERSION=\\\"${VERSION}\\\"" >>$CONFIG_MK
 echo "CPPFLAGS +=	-D_unused=\"__attribute__((__unused__))\"" >>$CONFIG_MK
 
 if [ -d .git ]; then
-	_gitrev=$(git rev-parse master)
+	_gitrev=$(git rev-parse --short master)
 	echo "CPPFLAGS += -DXBPS_GIT=\\\"${_gitrev}\\\"" >>$CONFIG_MK
 fi