Fixes issue #740

Shell --build has optional buildtrees

Closes #740

See merge request !986

README.rst: Add license badge

See merge request !1014

The queue shouldn't need to know about individual sources. This is in
line with _track() and _get_consistency().

Do not call fetch() for cached sources

See merge request !992

Add a `_GitSourceBase` class to act as the base class for deriving
source plugins that work with Git. The first user of this base class is
the `git` source plugin that's in core at the moment.

The contents of this base class is almost identical to the existing `GitSource`
class. The two notable differences are:

- the private methods now have leading underscores, as some were missing
  it originally
- the name of the class

Note that we are exposing a private member here as we expect it to move to a
separate package soon. See the following discussion for more details:
#739 (comment 124819869)

Fixes #739.

Expose base class for Git source plugins

Closes #739

See merge request !1019

Angelos Evripiotis authored Dec 19, 2018 and Angelos Evripiotis committed Dec 20, 2018

Now that we are also looking for WORKSPACE_PROJECT_FILE, we should add
it to the error message. While we're there, also mention the directory
we are looking in - this could help scripters be clear about what went
wrong.

The new error message looks like this:

    Error loading project: None of ['project.conf', '.bstproject.yaml']
    found in '/src/temp/blah' or any of its parent directories

Angelos Evripiotis authored Dec 19, 2018 and Angelos Evripiotis committed Dec 20, 2018

In the event that the project could not be found, stop BuildStream from
asking if the user would like to create a new project. Exit with error
instead, and give a hint to the user in case they're new.

As proposed on the mailing list here:
https://mail.gnome.org/archives/buildstream-list/2018-December/msg00082.html

The new interaction looks like this:

    $ bst show nonsuch.bst
    No project found. You can create a new project like so:

        bst init

    Error loading project: None of ['project.conf', '.bstproject.yaml']
    found in '/src/temp/blah' or any of its parent directories

Fixes #826