parse-remote: support default reflist in get_remote_merge_branch

Expand get_remote_merge_branch to compute the tracking branch to merge
when called without arguments (or only the remote name). This allows
"git pull --rebase" without arguments (default upstream branch) to
work with a rebased upstream. With explicit arguments it already worked.

Also add a test to check for this case.

Signed-off-by: Santi Béjar <santi@agolina.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-parse-remote.sh

@@ -232,7 +232,13 @@ get_remote_refs_for_fetch () {
 get_remote_merge_branch () {
 	case "$#" in
 	0|1)
-	    die "internal error: get-remote-merge-branch." ;;
+	    origin="$1"
+	    default=$(get_default_remote)
+	    test -z "$origin" && origin=$default
+	    curr_branch=$(git symbolic-ref -q HEAD)
+	    [ "$origin" = "$default" ] &&
+	    echo $(git for-each-ref --format='%(upstream)' $curr_branch)
+	    ;;
 	*)
 	    repo=$1
 	    shift

t/t5520-pull.sh

@@ -92,20 +92,34 @@ test_expect_success '--rebase with rebased upstream' '
 
 	git remote add -f me . &&
 	git checkout copy &&
+	git tag copy-orig &&
 	git reset --hard HEAD^ &&
 	echo conflicting modification > file &&
 	git commit -m conflict file &&
 	git checkout to-rebase &&
 	echo file > file2 &&
 	git commit -m to-rebase file2 &&
+	git tag to-rebase-orig &&
 	git pull --rebase me copy &&
 	test "conflicting modification" = "$(cat file)" &&
 	test file = $(cat file2)
 
 '
 
+test_expect_success '--rebase with rebased default upstream' '
+
+	git update-ref refs/remotes/me/copy copy-orig &&
+	git checkout --track -b to-rebase2 me/copy &&
+	git reset --hard to-rebase-orig &&
+	git pull --rebase &&
+	test "conflicting modification" = "$(cat file)" &&
+	test file = $(cat file2)
+
+'
+
 test_expect_success 'pull --rebase dies early with dirty working directory' '
 
+	git checkout to-rebase &&
 	git update-ref refs/remotes/me/copy copy^ &&
 	COPY=$(git rev-parse --verify me/copy) &&
 	git rebase --onto $COPY copy &&