fast-import: filemodify after M 040000 <tree> "" crashes

Until M 040000 <tree> "" syntax was introduced in commit 2794ad5
(fast-import: Allow filemodify to set the root, 2010-10-10), it
was impossible for the root entry to refer to an unloaded tree.
Update various functions to take that possibility into account.
Otherwise

	M 040000 <tree> ""
	M 100644 :1 "foo"

and similar commands (using D, C, or R after resetting the root
tree) segfault.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -1444,7 +1444,7 @@ static int tree_content_set(
 	const uint16_t mode,
 	struct tree_content *subtree)
 {
-	struct tree_content *t = root->tree;
+	struct tree_content *t;
 	const char *slash1;
 	unsigned int i, n;
 	struct tree_entry *e;
@@ -1468,6 +1468,9 @@ static int tree_content_set(
 	if (!slash1 && !S_ISDIR(mode) && subtree)
 		die("Non-directories cannot have subtrees");
 
+	if (!root->tree)
+		load_tree(root);
+	t = root->tree;
 	for (i = 0; i < t->entry_count; i++) {
 		e = t->entries[i];
 		if (e->name->str_len == n && !strncmp(p, e->name->str_dat, n)) {
@@ -1523,7 +1526,7 @@ static int tree_content_remove(
 	const char *p,
 	struct tree_entry *backup_leaf)
 {
-	struct tree_content *t = root->tree;
+	struct tree_content *t;
 	const char *slash1;
 	unsigned int i, n;
 	struct tree_entry *e;
@@ -1534,6 +1537,9 @@ static int tree_content_remove(
 	else
 		n = strlen(p);
 
+	if (!root->tree)
+		load_tree(root);
+	t = root->tree;
 	for (i = 0; i < t->entry_count; i++) {
 		e = t->entries[i];
 		if (e->name->str_len == n && !strncmp(p, e->name->str_dat, n)) {
@@ -1581,7 +1587,7 @@ static int tree_content_get(
 	const char *p,
 	struct tree_entry *leaf)
 {
-	struct tree_content *t = root->tree;
+	struct tree_content *t;
 	const char *slash1;
 	unsigned int i, n;
 	struct tree_entry *e;
@@ -1592,6 +1598,9 @@ static int tree_content_get(
 	else
 		n = strlen(p);
 
+	if (!root->tree)
+		load_tree(root);
+	t = root->tree;
 	for (i = 0; i < t->entry_count; i++) {
 		e = t->entries[i];
 		if (e->name->str_len == n && !strncmp(p, e->name->str_dat, n)) {
@@ -2056,13 +2065,16 @@ static uintmax_t do_change_note_fanout(
 		char *fullpath, unsigned int fullpath_len,
 		unsigned char fanout)
 {
-	struct tree_content *t = root->tree;
+	struct tree_content *t;
 	struct tree_entry *e, leaf;
 	unsigned int i, tmp_hex_sha1_len, tmp_fullpath_len;
 	uintmax_t num_notes = 0;
 	unsigned char sha1[20];
 	char realpath[60];
 
+	if (!root->tree);
+		load_tree(root);
+	t = root->tree;
 	for (i = 0; t && i < t->entry_count; i++) {
 		e = t->entries[i];
 		tmp_hex_sha1_len = hex_sha1_len + e->name->str_len;

t/t9300-fast-import.sh

@@ -928,6 +928,43 @@ test_expect_success \
 	 git diff-tree -C --find-copies-harder -r N5^^ N5 >actual &&
 	 compare_diff_raw expect actual'
 
+test_expect_success \
+	'N: copy to root by id and modify' \
+	'echo "hello, world" >expect.foo &&
+	 echo hello >expect.bar &&
+	 git fast-import <<-SETUP_END &&
+	commit refs/heads/N7
+	committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+	data <<COMMIT
+	hello, tree
+	COMMIT
+
+	deleteall
+	M 644 inline foo/bar
+	data <<EOF
+	hello
+	EOF
+	SETUP_END
+
+	 tree=$(git rev-parse --verify N7:) &&
+	 git fast-import <<-INPUT_END &&
+	commit refs/heads/N8
+	committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+	data <<COMMIT
+	copy to root by id and modify
+	COMMIT
+
+	M 040000 $tree ""
+	M 644 inline foo/foo
+	data <<EOF
+	hello, world
+	EOF
+	INPUT_END
+	 git show N8:foo/foo >actual.foo &&
+	 git show N8:foo/bar >actual.bar &&
+	 test_cmp expect.foo actual.foo &&
+	 test_cmp expect.bar actual.bar'
+
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 ### series O
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