pull: use merge-base --fork-point when appropriate

Since commit d96855ff (merge-base: teach "--fork-point" mode, 2013-10-23)
we can replace a shell loop in git-pull with a single call to
git-merge-base.  So let's do so.
Signed-off-by: NJohn Keeping <john@keeping.me.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-pull.sh

@@ -229,15 +229,7 @@ test true = "$rebase" && {
 	test -n "$curr_branch" &&
 	. git-parse-remote &&
 	remoteref="$(get_remote_merge_branch "$@" 2>/dev/null)" &&
-	oldremoteref="$(git rev-parse -q --verify "$remoteref")" &&
-	for reflog in $(git rev-list -g $remoteref 2>/dev/null)
-	do
-		if test "$reflog" = "$(git merge-base $reflog $curr_branch)"
-		then
-			oldremoteref="$reflog"
-			break
-		fi
-	done
+	oldremoteref=$(git merge-base --fork-point "$remoteref" $curr_branch)
 }
 orig_head=$(git rev-parse -q --verify HEAD)
 git fetch $verbosity $progress $dry_run $recurse_submodules --update-head-ok "$@" || exit 1