Merge branch 'jk/maint-1.6.3-checkout-unborn' into maint

* jk/maint-1.6.3-checkout-unborn:
  checkout: do not imply "-f" on unborn branches

builtin-checkout.c

@@ -402,7 +402,9 @@ static int merge_working_tree(struct checkout_opts *opts,
 		topts.dir = xcalloc(1, sizeof(*topts.dir));
 		topts.dir->flags |= DIR_SHOW_IGNORED;
 		topts.dir->exclude_per_dir = ".gitignore";
-		tree = parse_tree_indirect(old->commit->object.sha1);
+		tree = parse_tree_indirect(old->commit ?
+					   old->commit->object.sha1 :
+					   (unsigned char *)EMPTY_TREE_SHA1_BIN);
 		init_tree_desc(&trees[0], tree->buffer, tree->size);
 		tree = parse_tree_indirect(new->commit->object.sha1);
 		init_tree_desc(&trees[1], tree->buffer, tree->size);
@@ -541,14 +543,6 @@ static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
 		parse_commit(new->commit);
 	}
 
-	if (!old.commit && !opts->force) {
-		if (!opts->quiet) {
-			warning("You appear to be on a branch yet to be born.");
-			warning("Forcing checkout of %s.", new->name);
-		}
-		opts->force = 1;
-	}
-
 	ret = merge_working_tree(opts, &old, new);
 	if (ret)
 		return ret;

t/t2015-checkout-unborn.sh

@@ -0,0 +1,40 @@
+#!/bin/sh
+
+test_description='checkout from unborn branch protects contents'
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	mkdir parent &&
+	(cd parent &&
+	 git init &&
+	 echo content >file &&
+	 git add file &&
+	 git commit -m base
+	) &&
+	git fetch parent master:origin
+'
+
+test_expect_success 'checkout from unborn preserves untracked files' '
+	echo precious >expect &&
+	echo precious >file &&
+	test_must_fail git checkout -b new origin &&
+	test_cmp expect file
+'
+
+test_expect_success 'checkout from unborn preserves index contents' '
+	echo precious >expect &&
+	echo precious >file &&
+	git add file &&
+	test_must_fail git checkout -b new origin &&
+	test_cmp expect file &&
+	git show :file >file &&
+	test_cmp expect file
+'
+
+test_expect_success 'checkout from unborn merges identical index contents' '
+	echo content >file &&
+	git add file &&
+	git checkout -b new origin
+'
+
+test_done