Some refactors about GetMergeBase (#36186)

Maybe fix #32018

- Use `gitrepo.GetMergeBase` method instead of other two
implementations.
- Add `FetchRemoteCommit` so that we don't need to add many `remote` to
the git repository to avoid possible git lock conflicts. A lock will
start when invoke the function, it will be invoked when cross-repository
comparing. The head repository will fetch the base repository's base
commit id. In most situations, it should lock the fork repositories so
that it should not become a bottleneck.
- Improve `GetCompareInfo` to remove unnecessarily adding remote.
- Remove unnecessary parameters of `SignMerge`.

services/pull/check.go

@@ -238,7 +238,7 @@ func isSignedIfRequired(ctx context.Context, pr *issues_model.PullRequest, doer
 	}
 	defer closer.Close()
 
-	sign, _, _, err := asymkey_service.SignMerge(ctx, pr, doer, gitRepo, pr.BaseBranch, pr.GetGitHeadRefName())
+	sign, _, _, err := asymkey_service.SignMerge(ctx, pr, doer, gitRepo)
 
 	return sign, err
 }

services/pull/merge_prepare.go

@@ -19,6 +19,7 @@ import (
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/git/gitcmd"
+	"code.gitea.io/gitea/modules/gitrepo"
 	"code.gitea.io/gitea/modules/log"
 	asymkey_service "code.gitea.io/gitea/services/asymkey"
 )
@@ -105,7 +106,7 @@ func createTemporaryRepoForMerge(ctx context.Context, pr *issues_model.PullReque
 	mergeCtx.sig = doer.NewGitSig()
 	mergeCtx.committer = mergeCtx.sig
 
-	gitRepo, err := git.OpenRepository(ctx, mergeCtx.tmpBasePath)
+	gitRepo, err := gitrepo.OpenRepository(ctx, pr.BaseRepo)
 	if err != nil {
 		defer cancel()
 		return nil, nil, fmt.Errorf("failed to open temp git repo for pr[%d]: %w", mergeCtx.pr.ID, err)
@@ -113,7 +114,7 @@ func createTemporaryRepoForMerge(ctx context.Context, pr *issues_model.PullReque
 	defer gitRepo.Close()
 
 	// Determine if we should sign
-	sign, key, signer, _ := asymkey_service.SignMerge(ctx, mergeCtx.pr, mergeCtx.doer, gitRepo, "HEAD", trackingBranch)
+	sign, key, signer, _ := asymkey_service.SignMerge(ctx, pr, doer, gitRepo)
 	if sign {
 		mergeCtx.signKey = key
 		if pr.BaseRepo.GetTrustModel() == repo_model.CommitterTrustModel || pr.BaseRepo.GetTrustModel() == repo_model.CollaboratorCommitterTrustModel {

services/pull/pull.go

@@ -521,14 +521,7 @@ func checkIfPRContentChanged(ctx context.Context, pr *issues_model.PullRequest,
 	}
 	defer cancel()
 
-	tmpRepo, err := git.OpenRepository(ctx, prCtx.tmpBasePath)
-	if err != nil {
-		return false, "", fmt.Errorf("OpenRepository: %w", err)
-	}
-	defer tmpRepo.Close()
-
-	// Find the merge-base
-	mergeBase, _, err = tmpRepo.GetMergeBase("", "base", "tracking")
+	mergeBase, err = gitrepo.MergeBase(ctx, pr.BaseRepo, pr.BaseBranch, pr.GetGitHeadRefName())
 	if err != nil {
 		return false, "", fmt.Errorf("GetMergeBase: %w", err)
 	}