Refactor getpatch/getdiff functions and remove unnecessary fallback (#32817)

Extract from #32786 

`git diff a..b` is equal to `git diff a b` which is different from `git
diff a...b`. For pull request, we should always 

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

3 files changed, 24 insertions, 67 deletions

diff --git a/modules/git/repo_compare.go b/modules/git/repo_compare.go
index 16fcdcf4c8..877a7ff3b8 100644
--- a/modules/git/repo_compare.go
+++ b/modules/git/repo_compare.go
@@ -233,72 +233,34 @@ func parseDiffStat(stdout string) (numFiles, totalAdditions, totalDeletions int,
 	return numFiles, totalAdditions, totalDeletions, err
 }
 
-// GetDiffOrPatch generates either diff or formatted patch data between given revisions
-func (repo *Repository) GetDiffOrPatch(base, head string, w io.Writer, patch, binary bool) error {
-	if patch {
-		return repo.GetPatch(base, head, w)
-	}
-	if binary {
-		return repo.GetDiffBinary(base, head, w)
-	}
-	return repo.GetDiff(base, head, w)
-}
-
 // GetDiff generates and returns patch data between given revisions, optimized for human readability
-func (repo *Repository) GetDiff(base, head string, w io.Writer) error {
+func (repo *Repository) GetDiff(compareArg string, w io.Writer) error {
 	stderr := new(bytes.Buffer)
-	err := NewCommand(repo.Ctx, "diff", "-p").AddDynamicArguments(base + "..." + head).
+	return NewCommand(repo.Ctx, "diff", "-p").AddDynamicArguments(compareArg).
 		Run(&RunOpts{
 			Dir:    repo.Path,
 			Stdout: w,
 			Stderr: stderr,
 		})
-	if err != nil && bytes.Contains(stderr.Bytes(), []byte("no merge base")) {
-		return NewCommand(repo.Ctx, "diff", "-p").AddDynamicArguments(base, head).
-			Run(&RunOpts{
-				Dir:    repo.Path,
-				Stdout: w,
-			})
-	}
-	return err
 }
 
 // GetDiffBinary generates and returns patch data between given revisions, including binary diffs.
-func (repo *Repository) GetDiffBinary(base, head string, w io.Writer) error {
-	stderr := new(bytes.Buffer)
-	err := NewCommand(repo.Ctx, "diff", "-p", "--binary", "--histogram").AddDynamicArguments(base + "..." + head).
-		Run(&RunOpts{
-			Dir:    repo.Path,
-			Stdout: w,
-			Stderr: stderr,
-		})
-	if err != nil && bytes.Contains(stderr.Bytes(), []byte("no merge base")) {
-		return NewCommand(repo.Ctx, "diff", "-p", "--binary", "--histogram").AddDynamicArguments(base, head).
-			Run(&RunOpts{
-				Dir:    repo.Path,
-				Stdout: w,
-			})
-	}
-	return err
+func (repo *Repository) GetDiffBinary(compareArg string, w io.Writer) error {
+	return NewCommand(repo.Ctx, "diff", "-p", "--binary", "--histogram").AddDynamicArguments(compareArg).Run(&RunOpts{
+		Dir:    repo.Path,
+		Stdout: w,
+	})
 }
 
 // GetPatch generates and returns format-patch data between given revisions, able to be used with `git apply`
-func (repo *Repository) GetPatch(base, head string, w io.Writer) error {
+func (repo *Repository) GetPatch(compareArg string, w io.Writer) error {
 	stderr := new(bytes.Buffer)
-	err := NewCommand(repo.Ctx, "format-patch", "--binary", "--stdout").AddDynamicArguments(base + "..." + head).
+	return NewCommand(repo.Ctx, "format-patch", "--binary", "--stdout").AddDynamicArguments(compareArg).
 		Run(&RunOpts{
 			Dir:    repo.Path,
 			Stdout: w,
 			Stderr: stderr,
 		})
-	if err != nil && bytes.Contains(stderr.Bytes(), []byte("no merge base")) {
-		return NewCommand(repo.Ctx, "format-patch", "--binary", "--stdout").AddDynamicArguments(base, head).
-			Run(&RunOpts{
-				Dir:    repo.Path,
-				Stdout: w,
-			})
-	}
-	return err
 }
 
 // GetFilesChangedBetween returns a list of all files that have been changed between the given commits
@@ -329,21 +291,6 @@ func (repo *Repository) GetFilesChangedBetween(base, head string) ([]string, err
 	return split, err
 }
 
-// GetDiffFromMergeBase generates and return patch data from merge base to head
-func (repo *Repository) GetDiffFromMergeBase(base, head string, w io.Writer) error {
-	stderr := new(bytes.Buffer)
-	err := NewCommand(repo.Ctx, "diff", "-p", "--binary").AddDynamicArguments(base + "..." + head).
-		Run(&RunOpts{
-			Dir:    repo.Path,
-			Stdout: w,
-			Stderr: stderr,
-		})
-	if err != nil && bytes.Contains(stderr.Bytes(), []byte("no merge base")) {
-		return repo.GetDiffBinary(base, head, w)
-	}
-	return err
-}
-
 // ReadPatchCommit will check if a diff patch exists and return stats
 func (repo *Repository) ReadPatchCommit(prID int64) (commitSHA string, err error) {
 	// Migrated repositories download patches to "pulls" location
diff --git a/modules/git/repo_compare_test.go b/modules/git/repo_compare_test.go
index 454ed6b9f8..25ee4c5198 100644
--- a/modules/git/repo_compare_test.go
+++ b/modules/git/repo_compare_test.go
@@ -28,7 +28,7 @@ func TestGetFormatPatch(t *testing.T) {
 	defer repo.Close()
 
 	rd := &bytes.Buffer{}
-	err = repo.GetPatch("8d92fc95^", "8d92fc95", rd)
+	err = repo.GetPatch("8d92fc95^...8d92fc95", rd)
 	if err != nil {
 		assert.NoError(t, err)
 		return
diff --git a/services/pull/patch.go b/services/pull/patch.go
index 36ca9dbdb6..13623d73c6 100644
--- a/services/pull/patch.go
+++ b/services/pull/patch.go
@@ -41,9 +41,19 @@ func DownloadDiffOrPatch(ctx context.Context, pr *issues_model.PullRequest, w io
 	}
 	defer closer.Close()
 
-	if err := gitRepo.GetDiffOrPatch(pr.MergeBase, pr.GetGitRefName(), w, patch, binary); err != nil {
-		log.Error("Unable to get patch file from %s to %s in %s Error: %v", pr.MergeBase, pr.HeadBranch, pr.BaseRepo.FullName(), err)
-		return fmt.Errorf("Unable to get patch file from %s to %s in %s Error: %w", pr.MergeBase, pr.HeadBranch, pr.BaseRepo.FullName(), err)
+	compareArg := pr.MergeBase + "..." + pr.GetGitRefName()
+	switch {
+	case patch:
+		err = gitRepo.GetPatch(compareArg, w)
+	case binary:
+		err = gitRepo.GetDiffBinary(compareArg, w)
+	default:
+		err = gitRepo.GetDiff(compareArg, w)
+	}
+
+	if err != nil {
+		log.Error("unable to get patch file from %s to %s in %s Error: %v", pr.MergeBase, pr.HeadBranch, pr.BaseRepo.FullName(), err)
+		return fmt.Errorf("unable to get patch file from %s to %s in %s Error: %w", pr.MergeBase, pr.HeadBranch, pr.BaseRepo.FullName(), err)
 	}
 	return nil
 }
@@ -354,7 +364,7 @@ func checkConflicts(ctx context.Context, pr *issues_model.PullRequest, gitRepo *
 		_ = util.Remove(tmpPatchFile.Name())
 	}()
 
-	if err := gitRepo.GetDiffBinary(pr.MergeBase, "tracking", tmpPatchFile); err != nil {
+	if err := gitRepo.GetDiffBinary(pr.MergeBase+"...tracking", tmpPatchFile); err != nil {
 		tmpPatchFile.Close()
 		log.Error("Unable to get patch file from %s to %s in %s Error: %v", pr.MergeBase, pr.HeadBranch, pr.BaseRepo.FullName(), err)
 		return false, fmt.Errorf("unable to get patch file from %s to %s in %s Error: %w", pr.MergeBase, pr.HeadBranch, pr.BaseRepo.FullName(), err)