Explicitly specify missing keys in lms

In a project stanza, missing some of the branch-group keys would mean that kdesrc-build/kde-builder will use the branch "whatever git clone will use by default". As lms is describing the kde projects, which use master as default branch, this would mean master in that places.

Specify these missing keys explicitly, so it would be easier to identify possible mistakes (for example, incorrectly specified branches), and simplify comparing with branch-rules.yml.