Specify base branch for flake8 comparison (#5796)

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...`.

0 files changed, 0 insertions, 0 deletions