Fix PR creation on forked repositories

[LordChunk]

Resolves #20475

[LordChunk]

@lunny It's not entirely clear to me what changes should still be made to this PR. Would you mind clarifying your previous comment?

[lunny]

@lunny It's not entirely clear to me what changes should still be made to this PR. Would you mind clarifying your previous comment?

The logic looks right. Could you have a test for that change?

[LordChunk]

I've added a test which should test the new logic I added. I found it quite complicated to work our exactly how the test suite works, since I could not get single tests to run without getting this error:

Running tool: /usr/local/go/bin/go test -timeout 30s -tags 'sqlite sqlite_unlock_notify' -run ^TestPullCreatePrFromBaseToFork$ code.gitea.io/gitea/tests/integration -v
Environment variable $GITEA_CONF not set, use default: tests/sqlite.ini

sqlite3 requires: import _ "github.com/mattn/go-sqlite3" or -tags sqlite,sqlite_unlock_notify

FAIL  code.gitea.io/gitea/tests/integration 0.069s

The test as I've set it up right now is about the best that I could figure out. In case it doesn't quite work as intended, I have added a description of what I tried to make in the comment above the newly added test.

[lunny]

You can use make test-sqlite#Testxxxx to run the test locally.

[LordChunk]

@lunny I've added a test which should cover the changes I've made. If you've got any comments or questions, please let me know 📨