fast-import: treat filemodify with empty tree as delete

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 4b825dc642cb6eb9a060e54bf8d69288fbee4904 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>