Add branch references to GFM

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Problem

There is no way to reference branches from within GFM

Proposal

Following the newly minted extensible reference pattern (#217614 (closed)), add support for referencing branches from within GFM via [branch: ...].

Acceptance

• Support base syntax
• Support for autocomplete branch name
• Follows standards introduced in #217614 (closed)

Permissions

• Should match the roles that have visibility to see repos