Update bug field repohook

Cusky now uses b/<id> instead of b:<id>, which is used by ChromeOS. We
need to update the repohook that checks the bug field to reflect this.

BUG=b/162410910
TEST=pre-upload_unittest.py and tested on this commit message
RELEASE_NOTE=None

Change-Id: Ic443c005247d43d212415d1aff18b274001af8e0
(cherry picked from commit 4c1ef0344060c0fc0af4994421a9b923c19a2aa5)
2 files changed