[bug 1108755] Add a git commit message linter

As mentioned in the comments of bug 1108755 as requirements,
the commit message linter does the following things:

1. Enforces max. 79 characters per line
2. Verifies the summary line has bug numbers in the right format
3. Does a bugzilla lookup for the bug and prints out the 'bug id',
   'summary' and 'assigned to' fields.
You should have `lint_commit_msg` return an int and then have the `sys.exit()` call here. That way the script doesn't short-circuit and it's easier to test.