cmd/go-contrib-init: match subdomain-less .gitcookies file

My `.gitcookies` file starts with ".googlesource.com", which
errored because it did not match "go.googlesource.com" or
"go-review.googlesource.com". Fix this by optionally matching on
"go.googlesource.com" or ".googlesource.com".

Fixes golang/go#20992.

Change-Id: I29d3c0b1e958382495a90502f280bdb52868c2c7
Reviewed-on: https://go-review.googlesource.com/48230
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed