[PATCH] fast-import: Allow filemodify to set the root

[David Barr <david.barr@cordelta.com>]

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 4b825dc642cb6eb9a060e54bf8d69288fbee4904 ""

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

	git read-tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904

Commit-message-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: David Barr <david.barr@cordelta.com>
---
 fast-import.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/fast-import.c b/fast-import.c
index 2317b0f..8f68a89 100644
--- a/fast-import.c
+++ b/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)
-- 
1.7.3.4.g45608.dirty