Activating a different environment while activated leaves unclean environment variables

[nickajacks1]

Given two environments FOO (which sets env var MY_ENV=hello) and BAR (which sets MY_ENV=goodbye"):

1. activate FOO
2. without deactivating FOO, activate BAR
3. Hermit prepends BAR/bin to PATH without first removing FOO/bin and creates the environment variable _HERMIT_OLD_MY_ENV=hello
4. run deactivate-hermit
5. Hermit removes BAR/bin from PATH and restores MY_ENV=hello.
6. Issue: we are no longer in an activated hermit env, but we still have FOO/bin on PATH and still have MY_ENV=hello. Carrying artifacts of a previously activated hermit env around could have odd consequences, especially if something like JAVA_HOME is left set.

I assume that keeping FOO/bin on PATH is intentional for the sake of convenience in nested environments. If that is indeed the case, then one way to resolve this may be to keep a stack of previously activated environments and either:

• change deactivate-hermit to only deactivate a single environment (sounds more annoying than useful)
• ensure that deactivate-hermit explicitly unwinds all previously activated environments

I also noticed that shell-hooks will always explicitly deactivate environments before activating a new one, making it impossible to encounter this issue.