abuild: switch to svn co instead of export

Some projects use the svn revision in the version number.
svn co brings back the revision number but increases the archive size.

abuild.in

@@ -1849,7 +1849,7 @@ snapshot() {
                         die "Missing svn! Install subverion to support svn export."
                 [ -n "$reporev" ] && local _rev="-r $reporev"
                 msg "Creating svn snapshot: $pkgname-$_version"
-                svn export $_rev $svnurl $pkgname-$_version || return 1
+                svn co $_rev $svnurl $pkgname-$_version || return 1
                 tar zcf $pkgname-$_version.$_format $pkgname-$_version || return 1
         fi
         # upload to defined distfiles url