fix(pull): recover PRs whose recorded merge commit never reached the base repo (#39831) (#39840)

Backport #39831 by @lunny

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
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

modules/git/repo_commit_test.go

@@ -70,6 +70,20 @@ func TestGetCommitWithBadCommitID(t *testing.T) {
 	assert.True(t, IsErrNotExist(err))
 }
 
+func TestIsCommitInRepo(t *testing.T) {
+	bareRepo1, err := OpenRepositoryLocal(t.Context(), filepath.Join(testReposDir, "repo1_bare"))
+	assert.NoError(t, err)
+	defer bareRepo1.Close()
+
+	exist, err := IsCommitInRepo(t.Context(), bareRepo1, "2839944139e0de9737a044f78b0e4b40d989a9e3")
+	assert.NoError(t, err)
+	assert.True(t, exist)
+
+	exist, err = IsCommitInRepo(t.Context(), bareRepo1, "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")
+	assert.NoError(t, err)
+	assert.False(t, exist)
+}
+
 func TestIsCommitInBranch(t *testing.T) {
 	bareRepo1Path := filepath.Join(testReposDir, "repo1_bare")
 	bareRepo1, err := OpenRepositoryLocal(t.Context(), bareRepo1Path)
@@ -83,6 +97,10 @@ func TestIsCommitInBranch(t *testing.T) {
 	result, err = IsCommitInBranch(t.Context(), bareRepo1, "2839944139e0de9737a044f78b0e4b40d989a9e3", "branch2")
 	assert.NoError(t, err)
 	assert.False(t, result)
+
+	result, err = IsCommitInBranch(t.Context(), bareRepo1, "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", "branch1")
+	assert.NoError(t, err)
+	assert.False(t, result)
 }
 
 func TestRepository_CommitsBetween(t *testing.T) {