When building rcm I discovered that POSIX-compliant bash leaks extra environment variables; in my case, `DIRSTACK`.

Fixed it here: https://github.com/thoughtbot/rcm/commit/e5a1db282189a6d2a5b54c2aa14d9250e5d6b48d

Not sure whether this is worth noting, especially since we advise against naming non-exported variables with all caps, but there is a bigger lesson here: using `/bin/sh` as the shebang means you have to test on at least POSIX sh and GNU bash in POSIX-compliant mode.