git-svn: always use --first-parent

git-svn: always use --first-parent

This makes git-svn unconditionally invoke git-log with --first-parent when
it is trying to discover its upstream subversion branch and collecting the
commit ids which should be pushed to it with dcommit. The reason for always
using --first-parent is to make git-svn behave in a predictable way when the
ancestry chain contains merges with other git-svn branches.

Since git-svn now always uses 'git-log --first-parent' there is no longer
any need for the --first-parent option to git-svn, so this is removed.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>