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>

git-pull.sh

@@ -124,7 +124,7 @@ orig_head=$(git rev-parse --verify HEAD 2>/dev/null)
 git fetch --update-head-ok "$@" || exit 1
 
 curr_head=$(git rev-parse --verify HEAD 2>/dev/null)
-if test "$curr_head" != "$orig_head"
+if test -n "$orig_head" && test "$curr_head" != "$orig_head"
 then
 	# The fetch involved updating the current branch.
 
@@ -172,7 +172,7 @@ esac
 
 if test -z "$orig_head"
 then
-	git update-ref -m "initial pull" HEAD $merge_head "" &&
+	git update-ref -m "initial pull" HEAD $merge_head "$curr_head" &&
 	git read-tree --reset -u HEAD || exit 1
 	exit
 fi

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 &&