maintner: fix un-initialized github panic

Previously processGithubIssueMutation called initGithub, but
processGithubMutation did not, so if your on-disk mutation file had
a GithubMutation before a GithubIssueMutation, c.github would be nil
and Initialize() would panic.

Call initGithub from processGithubMutation and add a test to protect
against regressions.

Change-Id: Ie706fa04cb8ea87c2e0259dbee024c0005f0523f
Reviewed-on: https://go-review.googlesource.com/41298
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed