gitify - In hook stubs, set GIT_CANONICAL_REPO_NAME.

The hook functions are called by multiple repos, and we may want to tweak the behavior within each repo. The GIT_CANONICAL_REPO_NAME provides a value like "civicrm-packages" or "civicrm-core".
Merge pull request #119 from totten/git-hooks-reponame

gitify - In hook stubs, set GIT_CANONICAL_REPO_NAME.