Fix parsing of anchors in Phriction document link syntax

Summary: Ref T4280. At some point (probably D15732) we started getting anchor parsing wrong. Just pop the anchor off before doing all the logic, then put it back on at the end.

Test Plan:
Tested various forms like:

[[ x ]]
[[ x | z ]]
[[ x#y | z ]]
[[ ./x#y | z ]]

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4280

Differential Revision: https://secure.phabricator.com/D16083