fix(pull): recover PRs whose recorded merge commit never reached the base repo (#39831)

When pushing the merge commit to the base repo fails (e.g.
non-fast-forward), the merge intent recorded by
https://github.com/go-gitea/gitea/pull/39442 is left behind.
`merged_commit_id` then points to a commit that does not exist in the
base repo, and `git branch --contains` fails with `no such commit`, so
the PR can never be checked or merged again.

- Clear the recorded merge intent if the push fails and the commit is
not on the base branch
- `IsCommitInBranch` now returns false for a missing commit, so
already-stuck PRs recover on the next check

Fixes https://github.com/go-gitea/gitea/issues/39824

_Written with Codet assistance._

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>

services/pull/merge_test.go

@@ -4,8 +4,12 @@
 package pull
 
 import (
+	"strings"
 	"testing"
 
+	issues_model "gitea.dev/models/issues"
+	"gitea.dev/models/unittest"
+	user_model "gitea.dev/models/user"
 	"gitea.dev/modules/git"
 
 	"github.com/stretchr/testify/assert"
@@ -137,3 +141,19 @@ func TestResolveMergeMessageTemplate(t *testing.T) {
 		assert.Equal(t, "rebase template", tmpl)
 	})
 }
+
+func TestMergeIntentWithMissingCommit(t *testing.T) {
+	require.NoError(t, unittest.PrepareTestDatabase())
+	ctx := t.Context()
+	pr := unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{ID: 2})
+	require.NoError(t, recordMergeIntent(ctx, pr, &user_model.User{ID: 2}, strings.Repeat("a", 40), false))
+
+	merged, err := hasPullRequestCommitBeenMerged(ctx, pr)
+	require.NoError(t, err)
+	assert.False(t, merged)
+
+	require.NoError(t, clearMergeIntent(ctx, pr))
+	pr = unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{ID: 2})
+	assert.Empty(t, pr.MergedCommitID)
+	assert.Zero(t, pr.MergedUnix)
+}