better support for Atlassian Stash

Atlassian Stash names bare repository directories using an integer ID from Stash's internal representation.  This poses two problems:

* Grit::Repo auto-detects bare repositories by the common use of a ".git" suffix (which is absent in the case of Atlassian Stash)
* The repository name used in the JSON post is meaningless to flow participants

I've addressed these problems in this pull request.  The diff should speak for itself.  The changes should not affect existing users -- except in the rare event that the environment variable STASH_HOOK_CALLBACK is set.  That's an exceedingly unlikely circumstance.