Fix two issues with directory searches on Windows

First, make sure we actually match /c/windows from the beginning, not
if it occurs in the middle of the path.

Second, make sure that directories containing the binaries are
searched first. Do that by using unshift (prepend) instead of push
(append).

Signed-off-by: Thiago Macieira <thiago@macieira.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>