fast-import: allow moving the root tree

Because fast-import.c::tree_content_remove does not check for the empty
path, it is not possible to move the root tree to a subdirectory.
Instead the error "Path  not in branch" is produced (note the double
space where the empty path has been inserted).

Fix this by explicitly checking for the empty path and handling it.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -1568,7 +1568,8 @@ static int tree_content_set(
 static int tree_content_remove(
 	struct tree_entry *root,
 	const char *p,
-	struct tree_entry *backup_leaf)
+	struct tree_entry *backup_leaf,
+	int allow_root)
 {
 	struct tree_content *t;
 	const char *slash1;
@@ -1583,6 +1584,12 @@ static int tree_content_remove(
 
 	if (!root->tree)
 		load_tree(root);
+
+	if (!*p && allow_root) {
+		e = root;
+		goto del_entry;
+	}
+
 	t = root->tree;
 	for (i = 0; i < t->entry_count; i++) {
 		e = t->entries[i];
@@ -1599,7 +1606,7 @@ static int tree_content_remove(
 				goto del_entry;
 			if (!e->tree)
 				load_tree(e);
-			if (tree_content_remove(e, slash1 + 1, backup_leaf)) {
+			if (tree_content_remove(e, slash1 + 1, backup_leaf, 0)) {
 				for (n = 0; n < e->tree->entry_count; n++) {
 					if (e->tree->entries[n]->versions[1].mode) {
 						hashclr(root->versions[1].sha1);
@@ -2188,7 +2195,7 @@ static uintmax_t do_change_note_fanout(
 			}
 
 			/* Rename fullpath to realpath */
-			if (!tree_content_remove(orig_root, fullpath, &leaf))
+			if (!tree_content_remove(orig_root, fullpath, &leaf, 0))
 				die("Failed to remove path %s", fullpath);
 			tree_content_set(orig_root, realpath,
 				leaf.versions[1].sha1,
@@ -2323,7 +2330,7 @@ static void file_change_m(struct branch *b)
 
 	/* Git does not track empty, non-toplevel directories. */
 	if (S_ISDIR(mode) && !memcmp(sha1, EMPTY_TREE_SHA1_BIN, 20) && *p) {
-		tree_content_remove(&b->branch_tree, p, NULL);
+		tree_content_remove(&b->branch_tree, p, NULL, 0);
 		return;
 	}
 
@@ -2384,7 +2391,7 @@ static void file_change_d(struct branch *b)
 			die("Garbage after path in: %s", command_buf.buf);
 		p = uq.buf;
 	}
-	tree_content_remove(&b->branch_tree, p, NULL);
+	tree_content_remove(&b->branch_tree, p, NULL, 1);
 }
 
 static void file_change_cr(struct branch *b, int rename)
@@ -2422,7 +2429,7 @@ static void file_change_cr(struct branch *b, int rename)
 
 	memset(&leaf, 0, sizeof(leaf));
 	if (rename)
-		tree_content_remove(&b->branch_tree, s, &leaf);
+		tree_content_remove(&b->branch_tree, s, &leaf, 1);
 	else
 		tree_content_get(&b->branch_tree, s, &leaf, 1);
 	if (!leaf.versions[1].mode)
@@ -2530,7 +2537,7 @@ static void note_change_n(struct branch *b, unsigned char *old_fanout)
 	}
 
 	construct_path_with_fanout(sha1_to_hex(commit_sha1), *old_fanout, path);
-	if (tree_content_remove(&b->branch_tree, path, NULL))
+	if (tree_content_remove(&b->branch_tree, path, NULL, 0))
 		b->num_notes--;
 
 	if (is_null_sha1(sha1))

t/t9300-fast-import.sh

@@ -1050,7 +1050,7 @@ cat >expect <<EOF
 :100755 100755 e74b7d465e52746be2b4bae983670711e6e66657 e74b7d465e52746be2b4bae983670711e6e66657 R100	newdir/exec.sh	sub/newdir/exec.sh
 :100644 100644 fcf778cda181eaa1cbc9e9ce3a2e15ee9f9fe791 fcf778cda181eaa1cbc9e9ce3a2e15ee9f9fe791 R100	newdir/interesting	sub/newdir/interesting
 EOF
-test_expect_failure \
+test_expect_success \
 	'M: rename root to subdirectory' \
 	'git fast-import <input &&
 	 git diff-tree -M -r M4^ M4 >actual &&