maintner: use p.c instead of p.gr.github.c in githubRepoPoller.syncIssues
maintner: use p.c instead of p.gr.github.c in githubRepoPoller.syncIssues

The githubRepoPoller type has a field c, which is a shortcut for
gr.github.c. Other githubRepoPoller methods use it, so start using
it in syncIssues as well.

This makes the code more consistent and easier to read.

Also remove a spurious period in GitHubRepo.newMutationFromIssue
documentation.

Change-Id: Iecebd2ba6c69e3479e8d627fea691fcf9415aa51
Reviewed-on: https://go-review.googlesource.com/c/160841
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build