Add get_issue_number function to webkitpy/common/checkout/scm/git.py

This CL adds a "git cl issue" function that returns the issue number if one is associated with a branch and "None" otherwise.

Also adds unit test located in webkitpy/common/checkout/scm/scm_unittest.py

BUG=625254

Review-Url: https://codereview.chromium.org/2129733002
Cr-Commit-Position: refs/heads/master@{#404065}
2 files changed