Don't detect long sentences which happen to have a colon in them as Git URIs

Summary:
Some very liberal code is currently trying to parse commit summaries aggressively as Git URIs, and leading to this:

Failed to parse URI "Summary: Ref T11137. This addresses three general issues:" as a Git URI.

Currently visible here:

https://secure.phabricator.com/rPHU8bb124c37de3dfa29fcd23d4be53a5696b705c81

Don't detect these as Git URIs.

Test Plan: Unit tests.

Reviewers: chad

Reviewed By: chad

Subscribers: 20after4

Maniphest Tasks: T11137

Differential Revision: https://secure.phabricator.com/D16103