New fast-import test case for valid tree sorting

New fast-import test case for valid tree sorting

The Git tree sorting convention is more complex than just the name,
it needs to include the mode too to make sure trees sort as though
their name ends with "/".

This is a simple test case that verifies fast-import keeps the tree
ordering correct after editing the same tree twice in a single
input stream.  A recent proposed patch series (that has not yet
been applied) will cause this test to fail, due to a bug in the
way the series handles sorting within the trees.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>