This change calls `git clean` appropriately for the checked-out
repository and any submodules when checking out or switching branches.

Motivation:

Consider the case where a submodule exists on branch-a but not branch-b.
When using `mode=full` and `method=copy`, a `source` directory will
exist and it will contain the repository at branch-a. If the next build
is for branch-b, the following commands will be executed:

1. git fetch -f -t <source> branch-b --progress
2. git reset --hard <tip of branch-b> --
3. git checkout -B branch-b
4. git submodule sync
5. git submodule update --init --recursive --force --checkout

Command 2 here is expected to reset the checkout to the state <tip of
branch-b>, however it will fail to remove the submodule directory and
will issue a warning like:

```
warning: unable to rmdir 'path/to/submodule': Directory not empty
```

This leaves the source directory in an unexpected state and may cause
errors in the build.

Implementation:

This change ensures that `git clean -f -f -d -x` is called on the
checkout and that `_cleanSubmodule()` is called to clean submodules
recursively also.

Note that `-f` is required twice as:
> Git will refuse to modify untracked nested git repositories (directories
> with a .git subdirectory) unless a second -f is given.
-- man git-clean

Alternatives:

Initially, the option to `git submodule deinit --all` before resetting
the state was considered but this would require re-cloning submodules in
the more common case where they exist on both the current and previous
build.

Change the filtering in `is_message_needed_by_props` to accept the
non-virtual builder name for a build.

Motivation

The [Virtual Builders
pattern](https://docs.buildbot.net/latest/manual/configuration/builders.html?highlight=virtual_builder_name#virtual-builders)
allows us to schedule multiple builds of the same source and builder
with different properties. We use this to split the testing and
packaging work over many workers.

The issue is that we would like to define a GitHubStatusPush reporter
for each of these builders, however, as the `virtual_builder_name` of the
builder is calculated at build time, we can't enumerate them during
configuration.

In #6993, the submodule
initialisation logic was fixed to respect the `shallow` option.

This change adds a flag to maintain the previous behviour which we need
for some of our builds.