fast-import: Allow filemodify to set the root

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 4b825dc642cb6eb9a060e54bf8d69288fbee4904 ""

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

	git read-tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904

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>