[commit] Make revision format in commit message configurable

[aik099]

The commit message is generated automatically (user can still change it) for cases, when committing merge result. In there each merged revision is merged in rXXXX format (e.g. r23343). This default format for mentioning revisions inside Subversion.

If such commit is imported into Phabricator (see http://phabricator.org/) then revision doesn't become a link to corresponding commit because Phabricator expects revision links to look like rABC23343, where ABC is repository callsign under which that commit was imported.

Propose to:

1. create the config setting called commit.revision_format of choice type with following options:
   • Subversion (e.g. r12345) - default
   • Phabricator (e.g. rABC12345)
2. when for given working copy the Subversion format is chosen, then do all as now
3. when for given working copy the Phabricator format is chosen, then:
   • look for .arcconfig file in the working copy root
   • if not found throw an exception
   • if found, but repository.callsign key isn't found then throw an exception
   • if found then use it's value for revision link building