Fix fast-forward PR bug

[ethantkoenig]

Fixes #1186 and #1964.

If a PR is fast-forwarded, use the last commit of the PR as the "merge commit".

This ignores a more subtle problem, which is that we determine the "merger" of a PR by the author of the "merge commit", not by the user who actually pushed that commit to origin (which is how Github does it). Fixing this would require a much larger refactor, so for now I decided to just do a quick fix for the more glaring problem.

[lunny]

Looks good. Do we have PR integration test?

[lafriks]

@lunny yes, we have https://github.com/go-gitea/gitea/blob/master/integrations/pull_merge_test.go#L35

[lunny]

@lafriks but the CI is not failed before this PR. So I think that test didn't hit the situation this PR wants to resolve. But anyhow LGTM.

[ethantkoenig]

@lunny This is difficult to write an integration test for, because the error occurred in a background thread. The error wouldn't cause a test to fail, since the test has no way of checking whether the error occurred.

[ashkulz]

Will this be backported to 1.1.x?

[lunny]

@ashkulz sure.

[Bwko]

LGTM