Fix pull request compare link

[ethantkoenig]

The link should be parentOwner/parent/compare/master...childOwner:master instead of parentOwner/parent/compare/master...parentOwner:master.

Without this fix, the owner of the child repo gets a 404 when they click on the compare-pull-request button on the parent repo's homepage.

[lunny]

.SignedUser maybe nil?

[lunny]

Could you add some integration tests

[ethantkoenig]

If .SignedUser is nil, then .PullRequestCtx.Allowed will be false, so we don't need to worry about that.

I agree that it would be nice to not have to rely on dependencies between variables like this, but for the purpose of this PR I don't think a large-scale refactor is in order.

[ethantkoenig]

It seems to me like an integration test for this would have to rely on finding and following the relevant link in the DOM. I'm a little nervous to add such a test, since it would be fragile if we ever change the underlying templates.

If you have a better approach for an integration test, please let me know.

[lafriks]

I think it is ok to relay on DOM for integration tests, if UI is changed tests just need to be updated.

[lunny]

LGTM

[lafriks]

LGTM