Right now git_cl.py inserts an extra colon if the bug prefix ends with
a colon. Fix this by detecting whether the bug prefix ends with a colon
or not, and only inserting the colon when necessary.

Bug: 1111955
Change-Id: I3db199435087abb17b9600fc208b11d2d8e6a615
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2333426Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>