Rename "commitType" to "symbolicType"

Summary:
Ref T2683. The old name was a bit confusing because it meant "the type of the thing the symbol represents": a "commit type" should logically always be "commit".

(Currently, this is only used to detect when we're looking at a tag.)

Test Plan: Looked at a tag. Looked at some other non-tag things. Browsed around, `grep`.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2683

Differential Revision: https://secure.phabricator.com/D9092