fast-import: Allow filemodify to set the root

v1.7.3-rc0~75^2 (Teach fast-import to import subtrees named by tree id,
2010-06-30) has a shortcoming - it doesn't allow the root to be set.
Extend this behaviour by allowing the root to be referenced as the
empty path, "".

For a command (like filter-branch --subdirectory-filter) that wants
to commit a lot of trees that already exist in the object db, writing
undeltified objects as loose files only to repack them later can
involve a significant amount of overhead.
(23% slow-down observed on Linux 2.6.35, worse on Mac OS X 10.6)

Fortunately we have fast-import (which is one of the only git commands
that will write to a pack directly) but there is not an advertised way
to tell fast-import to commit a given tree without unpacking it.

This patch changes that, by allowing

	M 040000 <tree id> ""

as a filemodify line in a commit to reset to a particular tree without
any need to parse it.  For example,

	M 040000 4b825dc642 ""

is a synonym for the deleteall command and the fast-import equivalent of

	git read-tree 4b825dc642

Signed-off-by: David Barr <david.barr@cordelta.com>
Commit-message-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Sverre Rabbelier <srabbelier@gmail.com>
Tested-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fast-import.txt

@@ -524,6 +524,9 @@ start with double quote (`"`).
 If an `LF` or double quote must be encoded into `<path>` shell-style
 quoting should be used, e.g. `"path/with\n and \" in it"`.
 
+Additionally, in `040000` mode, `<path>` may also be an empty string
+(`""`) to specify the root of the tree.
+
 The value of `<path>` must be in canonical form. That is it must not:
 
 * contain an empty directory component (e.g. `foo//bar` is invalid),

fast-import.c

@@ -1454,6 +1454,15 @@ static int tree_content_set(
 		n = slash1 - p;
 	else
 		n = strlen(p);
+	if (!slash1 && !n) {
+		if (!S_ISDIR(mode))
+			die("Root cannot be a non-directory");
+		hashcpy(root->versions[1].sha1, sha1);
+		if (root->tree)
+			release_tree_content_recursive(root->tree);
+		root->tree = subtree;
+		return 1;
+	}
 	if (!n)
 		die("Empty path component found in input");
 	if (!slash1 && !S_ISDIR(mode) && subtree)

t/t9300-fast-import.sh

@@ -874,6 +874,27 @@ test_expect_success \
 	 git diff-tree -C --find-copies-harder -r N4^ N4 >actual &&
 	 compare_diff_raw expect actual'
 
+test_expect_success \
+	'N: copy root directory by tree hash' \
+	'cat >expect <<-\EOF &&
+	:100755 000000 f1fb5da718392694d0076d677d6d0e364c79b0bc 0000000000000000000000000000000000000000 D	file3/newf
+	:100644 000000 7123f7f44e39be127c5eb701e5968176ee9d78b1 0000000000000000000000000000000000000000 D	file3/oldf
+	EOF
+	 root=$(git rev-parse refs/heads/branch^0^{tree}) &&
+	 cat >input <<-INPUT_END &&
+	commit refs/heads/N6
+	committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+	data <<COMMIT
+	copy root directory by tree hash
+	COMMIT
+
+	from refs/heads/branch^0
+	M 040000 $root ""
+	INPUT_END
+	 git fast-import <input &&
+	 git diff-tree -C --find-copies-harder -r N4 N6 >actual &&
+	 compare_diff_raw expect actual'
+
 test_expect_success \
 	'N: modify copied tree' \
 	'cat >expect <<-\EOF &&