allow pull --rebase on branch yet to be born

When doing a "pull --rebase", we check to make sure that the index and
working tree are clean. The index-clean check compares the index against
HEAD. The test erroneously reports dirtiness if we don't have a HEAD yet.

In such an "unborn branch" case, by definition, a non-empty index won't
be based on whatever we are pulling down from the remote, and will lose
the local change.  Just check if $GIT_DIR/index exists and error out.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-pull.sh

@@ -119,11 +119,19 @@ error_on_no_merge_candidates () {
 }
 
 test true = "$rebase" && {
-	git update-index --ignore-submodules --refresh &&
-	git diff-files --ignore-submodules --quiet &&
-	git diff-index --ignore-submodules --cached --quiet HEAD -- ||
-	die "refusing to pull with rebase: your working tree is not up-to-date"
-
+	if ! git rev-parse -q --verify HEAD >/dev/null
+	then
+		# On an unborn branch
+		if test -f "$GIT_DIR/index"
+		then
+			die "updating an unborn branch with changes added to the index"
+		fi
+	else
+		git update-index --ignore-submodules --refresh &&
+		git diff-files --ignore-submodules --quiet &&
+		git diff-index --ignore-submodules --cached --quiet HEAD -- ||
+		die "refusing to pull with rebase: your working tree is not up-to-date"
+	fi
 	oldremoteref= &&
 	. git-parse-remote &&
 	remoteref="$(get_remote_merge_branch "$@" 2>/dev/null)" &&

t/t5520-pull.sh

@@ -149,4 +149,15 @@ test_expect_success 'pull --rebase dies early with dirty working directory' '
 
 '
 
+test_expect_success 'pull --rebase works on branch yet to be born' '
+	git rev-parse master >expect &&
+	mkdir empty_repo &&
+	(cd empty_repo &&
+	 git init &&
+	 git pull --rebase .. master &&
+	 git rev-parse HEAD >../actual
+	) &&
+	test_cmp expect actual
+'
+
 test_done