fast-import: treat filemodify with empty tree as delete

Normal git processes do not allow one to build a tree with an empty
subtree entry without trying hard at it.  This is in keeping with the
general UI philosophy: git tracks content, not empty directories.

v1.7.3-rc0~75^2 (2010-06-30) changed that by making it easy to include
an empty subtree in fast-import's active commit:

	M 040000 4b825dc642 subdir

One can trigger this by reading an empty tree (for example, the tree
corresponding to an empty root commit) and trying to move it to a
subtree.  It is better and more closely analogous to 'git read-tree
--prefix' to treat such commands as requests to remove the subtree.

Noticed-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -2163,6 +2163,12 @@ static void file_change_m(struct branch *b)
 		p = uq.buf;
 	}
 
+	/* 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);
+		return;
+	}
+
 	if (S_ISGITLINK(mode)) {
 		if (inline_data)
 			die("Git links cannot be specified 'inline': %s",

t/t9300-fast-import.sh

@@ -817,6 +817,48 @@ test_expect_success \
 	 git diff-tree -C --find-copies-harder -r N4^ N4 >actual &&
 	 compare_diff_raw expect actual'
 
+test_expect_success \
+	'N: delete directory by copying' \
+	'cat >expect <<-\EOF &&
+	OBJID
+	:100644 000000 OBJID OBJID D	foo/bar/qux
+	OBJID
+	:000000 100644 OBJID OBJID A	foo/bar/baz
+	:000000 100644 OBJID OBJID A	foo/bar/qux
+	EOF
+	 empty_tree=$(git mktree </dev/null) &&
+	 cat >input <<-INPUT_END &&
+	commit refs/heads/N-delete
+	committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+	data <<COMMIT
+	collect data to be deleted
+	COMMIT
+
+	deleteall
+	M 100644 inline foo/bar/baz
+	data <<DATA_END
+	hello
+	DATA_END
+	C "foo/bar/baz" "foo/bar/qux"
+	C "foo/bar/baz" "foo/bar/quux/1"
+	C "foo/bar/baz" "foo/bar/quuux"
+	M 040000 $empty_tree foo/bar/quux
+	M 040000 $empty_tree foo/bar/quuux
+
+	commit refs/heads/N-delete
+	committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+	data <<COMMIT
+	delete subdirectory
+	COMMIT
+
+	M 040000 $empty_tree foo/bar/qux
+	INPUT_END
+	 git fast-import <input &&
+	 git rev-list N-delete |
+		git diff-tree -r --stdin --root --always |
+		sed -e "s/$_x40/OBJID/g" >actual &&
+	 test_cmp expect actual'
+
 test_expect_success \
 	'N: modify copied tree' \
 	'cat >expect <<-\EOF &&