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.go

@@ -415,8 +415,26 @@ func (repo *Repository) GetCommitsFromIDs(ctx context.Context, commitIDs []strin
 	return commits
 }
 
-// IsCommitInBranch check if the commit is on the branch
+// IsCommitInRepo checks if the commit exists in the repository.
+func IsCommitInRepo(ctx context.Context, repo gitrepo.RepositoryFacade, commitID string) (bool, error) {
+	// "--quiet" makes it exit with 1 and no output if the commit doesn't exist
+	_, _, err := gitcmd.NewCommand("rev-parse", "--verify", "--quiet").
+		AddDynamicArguments(commitID + "^{commit}").
+		WithRepo(repo).
+		RunStdString(ctx)
+	if gitcmd.IsErrorExitCode(err, 1) {
+		return false, nil
+	}
+	return err == nil, err
+}
+
+// IsCommitInBranch checks if the commit is on the branch.
+// It returns false (no error) if the commit doesn't exist or isn't on the branch.
 func IsCommitInBranch(ctx context.Context, repo gitrepo.RepositoryFacade, commitID, branch string) (r bool, err error) {
+	if exist, err := IsCommitInRepo(ctx, repo, commitID); err != nil || !exist {
+		return false, err
+	}
+
 	stdout, _, err := gitcmd.NewCommand("branch", "--contains").
 		AddDynamicArguments(commitID, branch).
 		WithRepo(repo).
@@ -424,7 +442,7 @@ func IsCommitInBranch(ctx context.Context, repo gitrepo.RepositoryFacade, commit
 	if err != nil {
 		return false, err
 	}
-	return len(stdout) > 0, err
+	return len(stdout) > 0, nil
 }
 
 // GetCommitBranchStart returns the commit where the branch diverged