make repo callsigns optional

Summary:
Ref T4245 Make repo callsigns optional
This is far from done and still very ugly. I'm just submitting it to check if i'm solving this in the right places.
Right now there's three places with duplicate code and building the identifierMap in the CommitQuery is very ugly.
If we only want to support this in the user frontend then i could hack it into the Markup rule itself and not touch the CommitQuery. Even uglier but more limited in scope...

Generally this approach will need a lot of "check this first and then try the other" in a few places.
I could move the Repository queries into a specialised PhabricatorRepositoryQuery method (withCallsignOrID) but i'm not sure about that.

Test Plan:

• phid.lookup works with R1 and rTEST (which is the same repo)
• R1 and rTEST euqally work in remarkup (tested in comments).
• Reviewed the following syntax also all works:

rTEST
rTESTd773137a7cb9
rTEST:d773137a7cb9
R1
R1:d773137a7cb9
d773137a7cb9
Restricted Repository
{rTESTd773137a7cb9}
{rTEST:d773137a7cb9}
{R1}
{R1:d773137a7cb9}
{d773137a7cb9}

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D11050