Give it a more specific name. This improves clarity of what it means
and how it's being used in the ThreadType method.

Even though the package is a work in progress, it's still helpful
for it to have a description.

Still manually crafted in order to prototype and learn
about this space. It's still not completely clear how to
best integrate patch sets vs multiple commits, the commit
messages, and how git client fits into the picture.

The end result can be seen at
https://dmitri.shuralyov.com/gpu/mtl/...$changes/1.

Also remove unused commit header from diffMtlCommit1.

Still hard-coded mock data for now.

Update shurcool user to dmitshur.

This way, we detect a bad request and return an appropriate error
before starting anything else.

It's needed because in some meta-services that unify multiple
underlying services, the ThreadType changes depending on the RepoSpec.

Similar to https://github.com/shurcooL/issuesapp/commit/60bf33177c6943f244f14f81f145bc8b6c63f6cc.

Also return no error and empty result in List, Count when repo spec
doesn't match.