git-pull: allow pulling into an empty repository

We used to complain that we cannot merge anything we fetched
with a local branch that does not exist yet.  Just treat the
case as a natural extension of fast forwarding and make the
local branch'es tip point at the same commit we just fetched.
After all an empty repository without an initial commit is an
ancestor of any commit.

[jc: I added a trivial test.  We've become sloppy but we should
 stick to the discipline of covering new behaviour with new
 tests. ]

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-pull.sh

@@ -44,10 +44,10 @@ do
 	shift
 done
 
-orig_head=$(git-rev-parse --verify HEAD) || die "Pulling into a black hole?"
+orig_head=$(git-rev-parse --verify HEAD 2>/dev/null)
 git-fetch --update-head-ok --reflog-action=pull "$@" || exit 1
 
-curr_head=$(git-rev-parse --verify HEAD)
+curr_head=$(git-rev-parse --verify HEAD 2>/dev/null)
 if test "$curr_head" != "$orig_head"
 then
 	# The fetch involved updating the current branch.
@@ -80,6 +80,11 @@ case "$merge_head" in
 	exit 0
 	;;
 ?*' '?*)
+	if test -z "$orig_head"
+	then
+		echo >&2 "Cannot merge multiple branches into empty head"
+		exit 1
+	fi
 	var=`git-repo-config --get pull.octopus`
 	if test -n "$var"
 	then
@@ -95,6 +100,13 @@ case "$merge_head" in
 	;;
 esac
 
+if test -z "$orig_head"
+then
+	git-update-ref -m "initial pull" HEAD $merge_head "" &&
+	git-read-tree --reset -u HEAD || exit 1
+	exit
+fi
+
 case "$strategy_args" in
 '')
 	strategy_args=$strategy_default_args

t/t5520-pull.sh

@@ -0,0 +1,33 @@
+#!/bin/sh
+
+test_description='pulling into void'
+
+. ./test-lib.sh
+
+D=`pwd`
+
+test_expect_success setup '
+
+	echo file >file &&
+	git add file &&
+	git commit -a -m original
+
+'
+
+test_expect_success 'pulling into void' '
+	mkdir cloned &&
+	cd cloned &&
+	git init-db &&
+	git pull ..
+'
+
+cd "$D"
+
+test_expect_success 'checking the results' '
+	test -f file &&
+	test -f cloned/file &&
+	diff file cloned/file
+'
+
+test_done
+