When the current working directory is the root of the build tree, copy
it instead of moving it, otherwise an exception will be raised when we
try to clean up the build tree later.
Noticed when unpacking the vim source tree from a mercurial checkout.
Signed-off-by: Jack Nagel <jacknagel@gmail.com>
ClosesHomebrew/homebrew#13689.
Signed-off-by: Max Howell <mxcl@me.com>
There are subtle distinctions between writable? and writable_real? we don't
understand precisely why we need this, but it fixes the bugs :/
