[shell] Override CDPATH in brew script.

[gregorynisbet-google]

The CDPATH environment variable can affect the behavior of cd, and cd takes the -P, -L and -e flags. (I didn't know about -e until looking at the source) Make quietcd more robust by setting the CDPATH to ''.

I tested this change by running the following commands from a nonstandard directory, which caused some of tcsh's and perl's dependencies to be recompiled.

$ [path to brew] install tcsh
$ [path to brew] install perl

Here's a link to the source code of cd in a mirror of the bash repo.

https://github.com/bminor/bash/blob/ec8113b9861375e4e17b3307372569d429dec814/builtins/cd.def#L267

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[gregorynisbet-google]

For task 7, I'm running the tests on Linux in a nonstandard checkout, and the same tests fail on trunk and with my patch.

For task 4, I'm not sure how to write a test that tests the brew program as a whole rather than being a Ruby unit test.

[Bo98]

Makes sense, thanks!

[Bo98]

Thank you for your first contribution to Homebrew!