do not overwrite untracked during merge from unborn branch

In case HEAD does not point to a valid commit yet, merge is
implemented as a hard reset. This will cause untracked files to be
overwritten.

Instead, assume the empty tree for HEAD and do a regular merge. An
untracked file will cause the merge to abort and do nothing. If no
conflicting files are present, the merge will have the same effect
as a hard reset.
Signed-off-by: NClemens Buchacher <drizzd@aon.at>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -231,6 +231,24 @@ static void save_state(void)
 		die("not a valid object: %s", buffer.buf);
 }
 
+static void read_empty(unsigned const char *sha1, int verbose)
+{
+	int i = 0;
+	const char *args[7];
+
+	args[i++] = "read-tree";
+	if (verbose)
+		args[i++] = "-v";
+	args[i++] = "-m";
+	args[i++] = "-u";
+	args[i++] = EMPTY_TREE_SHA1_HEX;
+	args[i++] = sha1_to_hex(sha1);
+	args[i] = NULL;
+
+	if (run_command_v_opt(args, RUN_GIT_CMD))
+		die("read-tree failed");
+}
+
 static void reset_hard(unsigned const char *sha1, int verbose)
 {
 	int i = 0;
@@ -979,7 +997,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			die("%s - not something we can merge", argv[0]);
 		update_ref("initial pull", "HEAD", remote_head->sha1, NULL, 0,
 				DIE_ON_ERR);
-		reset_hard(remote_head->sha1, 0);
+		read_empty(remote_head->sha1, 0);
 		return 0;
 	} else {
 		struct strbuf merge_names = STRBUF_INIT;

t/t7607-merge-overwrite.sh

@@ -84,4 +84,20 @@ test_expect_success 'will not overwrite removed file with staged changes' '
 	test_cmp important c1.c
 '
 
+cat >expect <<\EOF
+error: Untracked working tree file 'c0.c' would be overwritten by merge.
+fatal: read-tree failed
+EOF
+
+test_expect_success 'will not overwrite untracked file on unborn branch' '
+	git reset --hard c0 &&
+	git rm -fr . &&
+	git checkout --orphan new &&
+	cp important c0.c &&
+	test_must_fail git merge c0 2>out &&
+	test_cmp out expect &&
+	test_path_is_missing .git/MERGE_HEAD &&
+	test_cmp important c0.c
+'
+
 test_done