This is guaranteed to be called only once for a given session once the
sources are known to be Consistency.CACHED, if source tracking is enabled
in the session for this source, then this will only be called if the
sources become cached after tracking completes.

 o Unlisted submodule warning

   Now the git plugin will issue a configurable warning if a submodule
   exists and is used (checking out the submodule is not disabled),
   but is not specified in the source configuration.

 o Invalid submodule warning

   Now the git source plugin will issue a warning if the configuration
   specified a submodule which does not exist in the underlying git repository.

As a side effect, this patch also changes the flow control of the git
plugin such that submodules which are explicitly set to not be
checked out, are also not fetched but instead ignored completely.

Now that we have Source.validate_cache(), this is a better place to emmit
the ref-not-in-track warning, since it will be emmitted at the earliest
opportunity and not only at Source.stage() or Source.init_workspace().

This also allows us to remove the `self.tracked` local state, and cleanup
some convoluted calling paths, removing some unnecessary parameters from
the usual codepaths and making the plugin overall more readable.

A function for removing paths from a git repo, can also
be used to remove submodules.

 o Test that it is not triggered in show before fetch, because we
   don't know about the unlisted submodules yet

 o Test that it is triggered by a fetch command

 o Test that it is triggered by `show` after having completed a
   fetch command, since now we have the repository and know about
   the unlisted submodule

 o Test all of this under warning or error conditions (parameterized
   for fatal-warnings)

   o Test that it is not triggered in show before fetch, because we
     don't know the submodules yet so we cannot know if they are
     valid or not.

   o Test that it is triggered by a fetch command

   o Test that it is triggered by `show` after having completed a
     fetch command, since now we have the repository and know which
     specified submodules are invalid

   o Test all of this under warning or error conditions (parameterized
     for fatal-warnings)

Implement submodule warnings

See merge request !996

Fix bst source-checkout not working with open workspaces

Closes #807

See merge request !997

This allows specifying whether or not the code believes a None
value is acceptable if explicitly expressed to be None in the YAML.

A previous patch 3ba544b8 changed
the behavior to accept None for all values across the board, along
with allowing explicitly setting the `default_value` to `None` which
caused the code to be much more readable (this was the main motivation
of the patch, but it had the side effect of allowing None for everything).

In the majority of cases we load YAML however, it either has a value or
it is not specified in the YAML, and None is hardly ever acceptable
to be explicitly specified (it may be in the case you want to override
something with nothing in the YAML, but it is the edge case).

This had the side effect of causing unexpected crashes where the
YAML specifies an empty dictionary for instance.

Instead of forcing the caller to handle a possible None value, give
the choice to the caller if they really want to accept a None value,
and raise the LoadError otherwise.

This fixes issue #803, which is a crash due to receival of an
unexpected None value through _yaml.node_get()

Allow plugins to conveniently leverage the new `allow_none` parameter,
and have Plugin.node_get_member() raise a LoadError in the usual
case where None is not an acceptable value to be explicitly set in
the user provided YAML.

This adds a regression test for issue #803

Only optionally allow None values in user provided YAML

Closes #803

See merge request !999

They are too specific to be included by default

Recommendation is if you are building in Linux is to use the
ones begin used in the freedesktop-sdk project, for example

See #645

Remove default strip-commands

Closes #645

See merge request !987

When using aliases there are multiple remotes used in the cache
repository. When fetching, tags are not updated if the were previously
fetched from a different remote. Commits that not in a branch and only
tagged do not get fetched if the tag is not fetched.

Fixes #812

Force updating tags when fetching git repository

Closes #812

See merge request !1000

i.e. with a given directory and filename, check parent directories until
either a directory with the filename is found, or you reach the root of
the filesystem.

This is a part of #222

Changes to _context.py:
* Context has been extended to contain a WorkspaceProjectCache, as there
  are times when we want to use it before a Workspaces can be initialised
  (looking up a WorkspaceProject to find the directory that the project is
  in)

Changes to _stream.py:
* Removed staging the elements from workspace_open() and workspace_reset()

Changes in _workspaces.py:
* A new WorkspaceProject contains all the information needed to refer back
  to a project from its workspace (currently this is the project path and
  the element used to create this workspace)
* This is stored within a new WorkspaceProjectCache object, which keeps
  WorkspaceProjects around so they don't need to be loaded from disk
  repeatedly.
* Workspaces has been extended to contain the WorkspaceProjectCache, and
  will use it when opening and closing workspaces.
* Workspaces.create_workspace has been extended to handle the staging of
  the element into the workspace, in addition to creating the equivalent
  WorkspaceProject file.

This is a part of #222

This is a part of #222

This is a part of #222

This is a part of #222

This is a part of #222

cli: Interactively warn if the user is trying to close the workspace they're using to load the project

This involves changes in:
* _stream.py:
  * Add the helper Stream.workspace_is_required()
* userconfig.yaml:
  * Add a default value for prompt.really-workspace-close-project-inaccessible
* _context.py:
  * Load the prompt 'really-workspace-close-project-inaccessible' from
    user config.
* cli.py:
  * If buildstream is invoked interactively, prompt the user to confirm
    that they want to close the workspace they're using to load this
    project.

This is a part of #222

This is a part of #222

This is a part of #222

Support invoking buildstream from a workspace outside a project

See merge request !924

_frontend/cli.py: Added helper function for walking through dirs to return files

data/projectconfig.yaml: Added new variable for `default: target-element`

_project.py: Added config variable to node_validate

Added supporting folders