Internally we work against a branch named 'llnl/develop', which
mirrors the public repository's `develop` branch.
It's useful to be able to run flake8 on our changes, using
`llnl/develop` as the base branch instead of `develop`.
Internally the flake8 subcommand generates the list of changes files
using a hardcoded range of `develop...`.
This makes the base of that range a command line option, with a
default of `develop`.
That lets us do this:
```
spack flake8 --base llnl/develop
```
which uses a range of `llnl/develop...`.
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