Fixes#1539
The branch matcher feature has been introduced in #1383, but the current
implementation was broken and doesn't work at all (#1539).
If my understanding is correct, there are two problems:
(1) The `GlobalCfg` has a default `Repo` instance which always matches
any repositries and branches. Therefore the branch matcher never be
functional.
(2) Validating base branches in
`DefaultPreWorkflowHooksCommandRunner.RunPreHooks()` implicitly assumed
that users customize `pre_workflow_hooks`, but the assumption isn't
always true because it defaults to empty.
For (1), I added a new method `MatchingRepo()` to `GlobalCfg` to check
`BranchMatches()` for a single `Repo` instance.
For (2), I moved validating branch to
`DefaultCommandRunner.validateCtxAndComment()`. Since the method has
already validated meta data of pull request, I think it's suitable place
to check base branches, but please let me know if there is anywhere more
suitable.
