Fix performance regression for partial commits

Fix performance regression for partial commits

When running "git commit paths" to create a partial commit, we
used to carefully build the temporary index so that we do not
lose the cached stat information.  The rewrite of the command in
C lost it by carelessly using read_tree().

This resurrects the earlier behaviour to keep the cached stat
information as much as possible by using one-tree merge logic.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>