pull: allow "git pull origin $something:$current_branch" into an unborn branch

Some misguided documents floating on the Net suggest this sequence:

    mkdir newdir && cd newdir
    git init
    git remote add origin $url
    git pull origin master:master

"git pull" has known about misguided "pull" that lets the underlying fetch
update the current branch for a long time.  It also has known about
"git pull origin master" into a branch yet to be born.

These two workarounds however were not aware of the existence of each
other and did not work well together.  This fixes it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5520-pull.sh

@@ -29,6 +29,18 @@ test_expect_success 'checking the results' '
 	diff file cloned/file
 '
 
+test_expect_success 'pulling into void using master:master' '
+	mkdir cloned-uho &&
+	(
+		cd cloned-uho &&
+		git init &&
+		git pull .. master:master
+	) &&
+	test -f file &&
+	test -f cloned-uho/file &&
+	test_cmp file cloned-uho/file
+'
+
 test_expect_success 'test . as a remote' '
 
 	git branch copy master &&